RFObjectPool refactoring as mentioned in #43.

RFObjectPool refactoring as mentioned in #43.
NSDateFormatter local pool realizations are removed from coders/decoders in ROADSerialization and replaced with RFObjectPool.
How about registering class for `key`? As I see we have to add object first, so we can use it in only once at a time in future, that's not convenient for some cases.