Implemented the ButtonDropdown component

Reference: http://twitter.github.io/bootstrap/components.html#buttonDropdowns

* Updates when new items are added/old items are removed;
* Changes the "disabled" class name when the `disabled` property is toggled;
* Invokes an action on the controller as specified by the `actionName` property;
* Allows the specifying of custom classes, such as `btn-danger`;
* `EXTEND_PROTOTYPES` compliant;
* Documented in the README.md for developers;