Better logging story

- MagicalRecord now correctly detects CocoaLumberjack.
- Updated MagicalRecord to use different log levels when
CocoaLumberjack is available.  When CocoaLumberjack is not available,
all log levels are logged via NSLog when DEBUG is defined and not
logged at all when DEBUG is undefined.
- Since MR_ENABLE_ACTIVE_RECORD_LOGGING doesn't easily work with
CocoaPods and because CocoaLumberjack is a better solution, I've
removed MR_ENABLE_ACTIVE_RECORD_LOGGING entirely.