Menu dropdowns usage is a bit broken

This PR fixes dropdown usage when using `maxDepth` and `onlyActiveBranch` options.

Basically you don't want the dropdown toggle anchor if the current most deep page also has children that will not be rendered.

Also the `renderDeepestMenu` method doesn't need to add dropdown classes at all, since it only renders a single level.

It may be worth noting that AFAIK Twitter Bootstrap doesn't handle more than one dropdown per navigation bar anyway, it's limited to 2 level nesting, so at the moment `setMaxDepth(1)` is the way to prevent the undesired markup. I'd like to know if and how you want me to address the problem.