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 DDLegacy.h shouldn't be importer in our new and shiny header.

What about:

```obj-c
// CocoaLumberjack.h
#ifndef DDLEGACY_ENABLED
#def DDLEGACY_ENABLED 0
#endif
```

```obj-c
#if DDLEGACY_ENABLED
#import "DDLegacy.h"
#endif
```

```
// DDLegacy.h
#def // everything that breaks projects and libraries that import DDLog.h instead of CocoaLumberjack.h (1.x projects)
```