Add Tests and Corrections for NGramIndexer._format_thing()
More Extensive Tests for indexers/interval.py

Basically to ensure coverage of all settings combinations in both the
vertical and horizontal indexers.
Add New Tests for the FilterByOffsetIndexer

Testing invalid quarterLength values.
Add Tests for AggregatedPieces

- One test for date ranges.
- One test for get_data() with incorrect arguments.
- Filed new issue (#241) to rewrite date-handling code in the future. It's
  not yet important enough to worry about, but we do have to do it!
Add a Test for Indexer

It fails because of a possible music21 bug, but I'm working on it!