Add class method proxy/protocol support

[keegnotrub]

We use inherited to know about Ruby classes that subclass NSObject. Coupling this with a hook to method_added allows us to bring Ruby instance methods into the Objective-C runtime. This is helpful when writing a Ruby class against an Objective-C protocol.

However, Objective-C protocols are also allowed to require class methods, so we should probably also hook into singleton_method_added to do the same there.

Note we will have to be careful to not proxy new as we define that ourselves.