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.
The pool behaves like this:  if there is no object for a particular key in a pool, and we have a class information and RFObjectPoolDefaultInitializerBlock, we can create a "default" object for this key and return it immediately. I agree, maybe it is not very clear behaviour, but there is a test for that.