Added Some Additional Tests to IndexedPiece

More to come...
Implement Caching NoteRestIndexer Results for IndexedPiece
Simple Implementation of "Stringifiers" for IndexedPiece

This includes __repr__, __str__, and __unicode__
Add Pylint Override in test_indexer.py
Add Fix and Test for Regression in IndexedPiece.get_data()

Related to commit 8ac425b80fbb55e6e133bbdd4d16887ae0e4ea65

When the NoteRestIndexer was run, any following Indexers wouldn't be.
Minor Optimization for indexer.series_indexer()

The pandas documentation says that union() is fast, so I hope they're
telling the truth.
Modified Indexing Functions to Include only "First" Events at an Offset

Also removed unused 3-part indexer tests, replacing with many fewer
useful ones.