Use NSUUID instead of CFUUIDRef

With this, we can get rid of the last `dealloc` method and the last `#import <CoreFoundation/CoreFoundation.h>` :smiley:

Thanks to my [NSUUID implementation](https://github.com/0xced/NSUUID) we can even transparently keep 10.7 compatibility even though the `NSUUID` class was only introduced in 10.8. Once we support 10.8 only, we can just get rid of the `NSUUID.m` file.