Add a cache in RKCompoundValueTransformer for source/destination class pairs, so that the subset of matching transformers does not have to be calculated every time.  This is a significant time savings while mapping.
Rename clearCache to invalidateCache, and cosmetic fix to if statement, per PR comments.
Merge pull request #14 from carllindberg/RKCompoundTransformerCache

Add a cache in RKCompoundValueTransformer