Implement time interval since 1970 value transformer. Lay groundwork for completing date formatter integration into the new architecture
Cleanup implementation to better match style of rest of the library
Remove dead static function
Drop old date formatter bits being replaced
Add tests for `NSNumberFormatter` implementation of `RKValueTransforming`
Implement `RKValueTransforming` for `NSDateFormatter` and `RKISO8601DateFormatter` to complete date integration into new architecture
Add comments for RKValueTransformers. Remove `numberToDateFormatter` in favor of the `timeIntervalSince1970ToDateValueTransformer`. Implement the object to collection transformer