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 think those are already handled by either:
* CocoaPods' `Pods-environment.h`:
```c
#define COCOAPODS_POD_AVAILABLE_CocoaLumberjack
#define COCOAPODS_VERSION_MAJOR_CocoaLumberjack 2
#define COCOAPODS_VERSION_MINOR_CocoaLumberjack 0
#define COCOAPODS_VERSION_PATCH_CocoaLumberjack 0
```
* Framework/static libraries' [Info.plist](https://github.com/CocoaLumberjack/CocoaLumberjack/blob/master/Framework/Desktop/Lumberjack/Lumberjack-Info.plist) (seems outdated right now).