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`.

Although that's a significant behavior change, it shouldn't break anything that was written correctly. In fact, it will probably fix some leaks.

As an aside, I didn't deprecate or remove the existing `-rac_textSignal` methods that we have for `NSControl` and `NSText`, mostly because I'm worried that Cocoa Bindings will be finicky and/or have bizarre edge cases. It seems like a good idea to have a reasonable fallback.

@Coneko Can you take a look at these changes and make sure they're sane?