Pass log context number to CocoaLumberjack if defined

Since MagicalRecord already has some reference to CocoaLumberjack, I decided to add an optional context number.

If CocoaLumberjack is pressent and the user defines the macro MR_LOG_CONTEXT with a number, this will be used as the logging context id. That way the user can reformat/filter/control the logs coming from MagicalRecord