Added Subclass Documentation/Headers

I've added some basic support for making it easier for devs to subclass MMDrawerController (#36) and handle some of the more specific requests that I've been seeing.

I'd like to wait until at least #20 is merged before merging this.

This would make #26 & #31 pretty easy for a developer.

This is a start to exposing some of the more "private" methods of MMDrawerController to a subclass. If anyone feels any of the exposed methods should be removed or some should be added feel free to chime in.