Modifications as requested by jspahrsummers:

- Marked UICollectionViewCell+RACSignalSupport.h as a public header.
- Adjusted indentation in UICollectionViewCell+RACSignalSupport.m.
- Changed to use dot syntax.
- Use NSStringFromClass instead of [Class description].
- Clarified a confusing comment.
- Simplified the expectations around invocationCount.