Allow stubbing of KVO objects

Hey Robb,

If I haven't already extended props, this is a great little tool, use it all the time :thumbsup: 

Issue is, though, that it can't stub KVO objects. Seems like using `object_getClass()` is too low level and ignores the [isa-swizzling](https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/KeyValueObserving/Articles/KVOImplementation.html) for a KVO object. Replacing this with `-class` takes this into account and *seems* to be fine, but for some reason I'm getting `EXC_BAD_ACCESS` gremlins. Maybe it's just something simple like some of the `object_getClass()` replacements is inappropriate?

Anyway, wanted to at least raise the issue and get your input. Then can get back to stubbing those RAC'd objects :wink: