[wip] rs-dropdown directive

This adds an `rs-dropdown` directive that can be used to implement a [dropdown element](http://canon-ui.com/ui-components/#dropdowns).

Markup example:

```html
<div rs-dropdown>
  <button class="rs-btn rs-btn-action rs-dropdown-toggle">
    <span class="rs-cog"></span>
    Actions
    <span class="rs-caret"></span>
  </button>
  <ul class="rs-dropdown-menu rs-pull-right hidden">
    <li class="rs-dropdown-item"><a class="rs-dropdown-link" ng-href="#/customers/edit/{{ currentCustomer.id }}">Edit Customer…</a>
    </li>
    <li class="rs-dropdown-item"><a class="rs-dropdown-link" href="#" ng-click="deleteCustomer(currentCustomer.id)">Delete Customer…</a></li>
  </ul>
</div>
```

Any `.rs-dropdown-toggle` element will work, so the directive should function with utility or primary dropdowns as well.

Left to do:
- [ ] Turn off all other menus when a menu is shown.
- [ ] Specs :point_up: