Fix CocoaLumberjack definition

Preprocessor definitions for CocoaLumberjack is move to .m file.

Problem description:
If project has a dependency to iTellAFriend and own global definition in
``` objective-c
//Constants.h
extern const int ddLogLevel;

//Constants.m
#ifdef DEBUG
static const int ddLogLevel = LOG_LEVEL_VERBOSE;
#else
static const int ddLogLevel = LOG_LEVEL_WARN;
#endif
```

Importing headers (Constants.h & iTellAFriend.h) will cause compilation error:
```
Redefinition of 'ddLogLevel' with a different type: 'int' vs 'const int'
```

Your definition of CocoaLumberjack is not described in README and as I understand added for own purposes.

So, I moved it to .m to avoid such possible conflicts.