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.
This seems like a good idea. Could be called `DDLegacy.h` and add all the necessary macros for apps/libraries not updated for 2.x.