Add UICollectionViewCell.rac_prepareForReuseSignal
- Removed UICollectionViewCell+RACSignalSupport.h from ReactiveCocoa.h because this category applies to iOS >= 6.0 and the apparent minimum version of iOS supported by ReactiveCocoa is iOS >= 5.0.
- Adjusted UICollectionViewCellRACSupportSpec.m to account for known cases where prepareForReuse is never called in some simulator/device configurations.
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.
Minor style tweaks