Rewritten and Simplified Metadata Implementation

Now we use a dict to store metadata settings.
Add Tests for IndexedPiece.__unicode__()
Mocked __init__()

Yeah, that's right.

Only sort of... I've improved the tests for IndexedPiece.get_data() by
attaching a MagicMock as the __init__ property of our fake Indexer types.
Update Dependencies in README
Revise Function/Method Documentation for Consistency

This way, even if we do have to change things again, at least it's all
the same.
Correction for BWV 2 Chorale

The music21 version is different from the Riemannschneider version that
Prof. Rusch used for the ground truth.