Use NS_ENUM and NS_OPTIONS for the log level and log flags

You can just pick fd9d3ed if you only want the enums/options.
This change does make CocoaLumberjack more Swift-friendly, but more work will be needed to make it viable.
That's a good thing! Unluckily I still have to maintain iOS 6 projects, so not using Swift just yet.

Do you think the proposed changes will play nicely with Swift (#299)?
Probably we will need a CocoaLumberjack.swift (#225).