[DON'T MERGE YET] Allow stubbing of KVO objects

**Not yet merge-able, need feedback** :wink: 

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 for a KVO object. Replacing this with `-class` takes this into account and *seems* to be fine, but for some reason I'm getting an `EXC_BAD_ACCESS` gremlin in `Class method stubs`. Maybe it's just something simple like one of the `object_getClass()` replacements is inappropriate?

Thought it might be quicker to get your input anyway, then can get back to stubbing those RAC'd objects :wink: