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.
I based it solely on the `LOG_LEVEL_ALL` value, which is a 32-bit integer. `NSUInteger` can be either 32-bit or 64-bit depending on the architecture.