Implemented different logLevel per each logger

Different log levels per loggers can be very useful. In all the projects I used CocoaLumberjack, I wanted to have different levels for each XCode configuration (Debug vsAdHoc vs AppStore), but also different for each logger.
Potential reasons:
- TTY logs are not needed on AppStore builds
- ASL are slow (even with the improvement from CocoaLumberjack), so I tend to set Warning level only for those in AppStore builds, as where the file logging will use a more permissive setting (like info)
- if you use a custom logger (like I do for Crashlytics logs), chance is you want a different log level there too