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.
But they won't be defined when using CocoaPods. Defining them would require another manual (error prone) update on every release.

We could keep it inside `#ifndef COCOAPODS` and update the Info.plist, as I guess that is what this looks up.