Wrap Cocoa Bindings with a RACBinding-based API

Fixes #452 and #361.

I also refactored `RACPropertySubject`, `RACBinding`, and the KVO equivalents to forward `completed` events. Infinite signals are a huge headache, and this means that any binding can now be manually terminated (which was previously not possible) by directly sending it `completed`.

@Coneko Can you take a look at these changes and make sure they're sane?