Added support for blocks in menu_item, to be more inline with link_to's ...

Added support for blocks in menu_item, to be more inline with link_to's capabilities.

This allows you to use blocks instead of the name parameter. For example:

    # inside menu_group
    <%= menu_item root_path do %>
      <i class="icon-home"></i> Home
    <% end %>
    # returns
    <li><a href="/">
      <i class="icon-home"></i> Home
    </a></li>

    # with no url works,
    <%= menu_item do %>
      <i class="icon-off></i> Off
    <% end %>
    # returns
    <li><a href="#">
      <i class="icon-off"></i> Off
    </a></li>