Add NSObject category with a convenient, lazy loaded KVOController

Hi,

Since the functionality of this component is "core", and I would like to use it much as I would use the default KVO functionality (without instantiating anything myself), I added this `NSObject` category to take that off my hands, and make the entry level one line shorter.

I haven't updated the read me to reflect that, I could do that if you guys need that.

Cheers,