Dropdown button and split button refactor + docs

In the ever expanding quest for docs comes the 3rd and final instalment of buttons!

Here are the changes:
* Move common dropdown state logic into `DropdownStateMixin` and simplify event logic.
* Removed transferPropsTo calls in favour of manually passing around props.
* Cleanup unit tests, make sure we are unmounting nodes that bind to the document.
* Added unit tests for `DropdownMenu`
* Added docs content and examples based on http://getbootstrap.com/components/#btn-dropdowns