Add failing test for KVO bug
Find the instance method rather than the immediate one on the class

This fixes the KVO bug where rac_signalForSelector methods wouldn't be invoked after the object is KVO'd.
Add test that signalForSelector's then KVOs then signalForSelector's again
Store the pre-swizzled class as an associated object on the object
Add test to check that the right signal is fired if there's two instances of the same class that add a signal for the same selector
Better name
Rearrange and add comment
Merge pull request #1281 from mattjgalloway/kvo_bug

Fixes KVO bug described in issue #1280