Adding attributes to <a> link in wp_nav_menu using custom Walker function

I am trying to generate a menu using wp_nav_menu. My expected output is this:

        <!-- Main Menu -->
<div class="navbar navbar-blue navbar-fixed-top" style="margin-top:74px">
    <div class="navbar-inner">
        <div class="container">


            <a class="btn btn-navbar" data-toggle="collapse" data-target=".nav-collapse">
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
            </a>
            <div class="nav-collapse collapse">

                                <ul class="nav">
                    <li class="divider-vertical"></li>
                    <li class="dropdown dropdown-hover">
                        <a href="dashboard.html?lang=en" class="dropdown-toggle" data-toggle="dropdown">Dashboard <b class="caret"></b></a>
                        <!-- Dropdown Menu -->
                        <ul class="dropdown-menu">
                            <li><a href="dashboard.html?lang=en">Dashboard #2</a></li>
                            <li><a href="dashboard_1.html?lang=en">Dashboard #1</a></li>
                        </ul> 
                        <!-- End Dropdown Menu  -->
                    </li>
                    <li class="divider-vertical"></li>
                    <li class="active"><a href="gallery.html?lang=en">Photo Gallery</a></li>
                    <li class="divider-vertical"></li>
                    <li class="dropdown dropdown-hover">
                        <a href="site-pages.html?lang=en" class="dropdown-toggle" data-toggle="dropdown">Site Pages <b class="caret"></b></a>
                        <!-- Dropdown Menu -->
                        <ul class="dropdown-menu">
                            <li><a href="site-pages.html?lang=en">View pages</a></li>
                            <li><a href="site-pages-add.html?lang=en">Add page</a></li>
                        </ul> 
                        <!-- End Dropdown Menu  -->
                    </li>
                    <li class="divider-vertical"></li>
                    <li class="dropdown dropdown-hover">
                        <a href="products.html?lang=en" class="dropdown-toggle" data-toggle="dropdown">eCommerce <b class="caret"></b></a> 
                        <!-- Dropdown Menu -->
                        <ul class="dropdown-menu">
                            <li class="nav-header">Catalog</li>
                            <li><a href="categories.html?lang=en">Categories</a></li>
                            <li><a href="products.html?lang=en">Products</a></li>
                            <li><a href="products-add.html?lang=en">Add product</a></li>
                            <li class="divider"></li>
                            <li><a href="orders.html?lang=en">Orders</a></li>
                        </ul> 
                        <!-- End Dropdown Menu  -->
                    </li>
                    <li class="divider-vertical"></li>
                    <li class="dropdown dropdown-hover">
                        <a href="clients.html?lang=en" class="dropdown-toggle" data-toggle="dropdown">Members <b class="caret"></b></a>
                        <!-- Dropdown Menu -->
                        <ul class="dropdown-menu">
                            <li><a href="clients.html?lang=en">Clients</a></li>
                            <li><a href="account.html?lang=en">Edit account</a></li>
                        </ul> 
                        <!-- End Dropdown Menu  -->
                    </li>
                    <li class="divider-vertical"></li>
                    <li class="dropdown dropdown-hover">
                        <a href="charts.html?lang=en" class="dropdown-toggle" data-toggle="dropdown">Other <b class="caret"></b></a>
                        <!-- Dropdown Menu -->
                        <ul class="dropdown-menu">
                            <li><a href="charts.html?lang=en">Charts</a></li>
                        </ul> 
                        <!-- End Dropdown Menu  -->
                    </li>
                    <li class="divider-vertical"></li>
                    <li class="dropdown dropdown-hover">
                        <a href="documentation.html?lang=en" class="dropdown-toggle" data-toggle="dropdown">Documentation <b class="caret"></b></a>
                        <!-- Dropdown Menu -->
                        <ul class="dropdown-menu">
                            <li><a href="documentation.html?lang=en">Overview</a></li>
                            <li><a href="documentation/features.html?lang=en">Features</a></li>
                            <li><a href="documentation/getting-started.html?lang=en">Getting Started</a></li>
                            <li><a href="documentation/customization-guide.html?lang=en">Customization Guide</a></li>
                            <li class="divider"></li>
                            <li class="nav-header">Powered by</li>
                            <li><a href="documentation/credits.html?lang=en">Credits</a></li>
                        </ul> 
                        <!-- End Dropdown Menu  -->
                    </li>
                    <li class="divider-vertical"></li>
                </ul>
            </div>
            <!--/.nav-collapse -->
        </div>
    </div>

</div>
<!-- End Main Menu -->

So far I am able to use a custom walker function to wp_nav_menu and achieved to generate almost all the html as above but I am unable to insert class=”dropdown-toggle” data-toggle=”dropdown” inside the tag. This will allow me to toggle the dropdown I guess (currently I am able to generate only the top menu. The submenu’s appear in html but they are not visible in browser.).

Here is my effort so far:

        <!-- Main Menu -->
<div class="navbar navbar-blue navbar-fixed-top" style="margin-top:74px">
    <div class="navbar-inner">
        <div class="container">


            <a class="btn btn-navbar" data-toggle="collapse" data-target=".nav-collapse">
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
            </a> 
<?php class My_Walker_Nav_Menu extends Walker_Nav_Menu {
function start_lvl(&$output, $depth) {
$indent = str_repeat("t", $depth);
$output .= "n$indent<ul class="dropdown-menu">n";
}
}
$defaults = array(
'theme_location'  => '',
'menu'            => '',
'container'       => 'div',
'container_class' => 'nav-collapse collapse',
'container_id'    => '',
'menu_class'      => 'nav',
'menu_id'         => '',
'echo'            => true,
'fallback_cb'     => 'wp_page_menu',
'before'          => '',
'after'           => '',
'link_before'     => '',
'link_after'      => '',
'items_wrap'      => '<ul id="%1$s" class="%2$s"><li class="divider-vertical"></li>%3$s</ul>',
'depth'           => 0,
'walker'          => new My_Walker_Nav_Menu
     );

     wp_nav_menu( $defaults );

      ?>
        </div>
    </div>

</div>
<!-- End Main Menu -->

Can somebody help me to modify my custom walker function so as to get the desired html output given above. Thanks in advance.