Replace Eyed3 with Mutagen

OK, this should solve #1, #27 and #38.

I rewrote ID3Manager and replaced EyeD3 with Mutagen.
    
ID3Manager is now called MetadataManager. Getter and setter methods with immediate persistence have been replaced with properties and a separate `save()` method.

The indexer uses docopt instead of its own argument parsing. I cherry-picked the commit by @felixhummel.

This pull request is probably not 100% ready for merge yet, as I didn't really test details of the indexer. And the models are not tested at all yet. Storing/updating metadata probably fails at the moment, as I've introduced a new `save()` method on the `MetadataManager`. I'll try to find the time to do that tomorrow.

If someone else wants to test this PR, feel free. Feedback is always welcome.