Update logging

RestKit relies too much on the underlying log framework while some people may want to use a different one.

This pull request abstracts the actual framework and adds support for NSLog-based logging as an example. Next [CocoaLumberjack](https://github.com/CocoaLumberjack/CocoaLumberjack) and [NBULog](https://github.com/CyberAgent/iOS-NBULog) support will be added.