Add NSLogger Support

Since MagicalRecord already has support for [CocoaLumberjack](https://github.com/robbiehanson/CocoaLumberjack), I 
added support for [NSLogger](https://github.com/fpillet/NSLogger). If NSLogger is present 
in the project, the MRLog macro is defined to log using NSLogger.

When installing NSLogger and MagicalRecord using [Cocoapods](http://cocoapods.org/), NSLogger
needs to be manually imported in Pods-prefix.pch in order to be
recognized by MagicalRecord. If using MagicalRecord as git submodule,
NSLogger support works out of the box.