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.
Note that using `NS_ENUM` and `NS_OPTIONS` for enums (or their CoreFoundation counterparts) map to Swift `enum`s or `RawOptionSetType`s, respectively. In fact, this is likely the reason they did it.

As for making it more Swift friendly, I do have a branch called `SwiftTest` where I'm experimenting with Swift code. One thing I did do was make a a version of `DDLog`'s logging function that accepts an `NSString` without using the three periods, as that version isn't handled by Swift's Objective C bridge right now.

Although additional changes regarding Swift compatibility can probably be handled in a different pull request.