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.
> ```c
#define COCOAPODS_POD_AVAILABLE_CocoaLumberjack
#define COCOAPODS_VERSION_MAJOR_CocoaLumberjack 2
#define COCOAPODS_VERSION_MINOR_CocoaLumberjack 0
#define COCOAPODS_VERSION_PATCH_CocoaLumberjack 0
```

No longer available with CocoaPods 0.34.1.