Submenus

add the posibility to use submenus in nav_bar

```
<%= nav_bar :fixed => :top, :brand => "BCN", :responsive => true do %>
  <%= menu_group do %>
    <%= menu_item "Home", root_path %>
      <%= drop_down_with_submenu "categories" do %>
        <%= drop_down_submenu 'Lan' do %>
          <%= menu_item 'Routers', routes_path %>
        <% end %>
      <% end %>
    <% end %>
  <% end %>
<% end %>
```