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.
Apple use naming like `reuseIdentifier` in its code for similar purpose, it's will be great to adopt such things for easier understanding of unprepared user. How do you think, maybe we can name it like:
```objc
- (id)objectForReuseIdentifier:(id<NSCopying>)reuseIdentifier;
```