Don't crash when `nil` error is provided to a value transformer
Remove `k` prefix on associated object key
Remove tests from removed date to number value transformer
Add RKValueTransformers to the ObjectMapping.h header
Add identity transformer, mutable value transformer, migrate date helper functions
Migrate legacy date formatter API to being backed by value transformers
Migrate internals of mapping engine over to value transformers
Add defaulting behavior so that `RKObjectMapping` falls back to the default value transformer when `nil`
Fix bug in object -> collection transformer
Add the mutable value transformer to the default compound transformer
Fix test exceptions by reimplementing date formatter reset logic
Fix a handful of issues with date formatting support
Add subclass check on mutable transformer
Additional adjustments to the date support