Update podspec to use CocoaLumberjack if it's available, and define the minimum deployment targets
Import CoreData into a few categories that use it that it was missing from
Reinstate the MR_LOGGING_ENABLED compile time checks for logging
Update the podspec to provide variants that enable logging.

I've put in a request for better support from CocoaPods for setups like ours.
Update logging documentation.

Closes #748.
Enable logging in the MagicalRecord Xcode project
Tidy up the format and logic flow in the save handlers
Rename logging enums :P (courtesy of @casademora to match 3adc1d82dcbb5e7ab6886664f44e3d7c94217e6b)