feat(dropdown): add dropdown and dropdown-menu directives

Initial work on some AngularJS ports of the dropdown modules.

As of yet, there is no real test suite, and testing has only been happening in
what I intend to be a future demo site for the AngularSemantic codebase.

Currently, transition support is experimental, using $animate's addClass
methods exclusively. I'd like to make this a bit more dynamic, and
additionally support jQuery animation functions if jQuery is detected -- But
for now, this is working nicely, I think.