- changes to mediafile.py. Provides a delete method which allows the user to clear current metadata information...

Add a change to mediafile.py. Provides a delete method which allows the user
      to remove all audio metadata from the mutagen call "delete"

example :
```python    
 mediafileobject.delete()
```

I am using this great mediafile.py library in other tagging project. Adding the delete method, yields a quick means to start from a fresh/clean audio file and build from there.