Tests: Add additional integration test for "intervals"

Wondering about a fix for the "interval n-grams" problems reported in #327,
I decided to try an additional integration test. Now I know that it's for sure
a problem with horizontal intervals!
Refactor: Remove unicode literals from test_interval_indexer.py

Positively barelling toward Python 3!
New: Add 'horiz_attach_later' Setting for HorizontalIntervalIndexer

In VIS 1, all horizontal intervals would be recorded at the offset of the later
note in the interval. In early VIS 2, this was changed to the offset of the
earlier note in the interval. Now there is a choice, defaulting to the earlier,
which more closely reflects the behaviour of nearly every other analyzer.

Related to #298 and #327
Fix: the "interval_ngrams" helper methods in WorkflowManager

Following through with the issues arising from #298 and #327, this simply
involved adding the 'horiz_attach_later' setting to the calls to the
interval indexers.
Fixed Tests: WorkflowManager integration 9a, 9b, 9c was loading the wrong piece

This happened after 20-Aug-2014 but before 03-Oct-2014, but I can't find the
commit that caused it. No matter---it's fixedish now.

Related to #idkanymore
Refactor: use pickled "expecteds" for "interval n-gram" integration tests

I was having some problems getting the CSV files to import consistently,
and it seems like the next-best format is pickle. The pandas documentation
says they should be forward-compatible too, so I hope that's the case.

Working toward #328
Refactor: Add assertDataFramesEqual() to "interval ngram" integration tests

This is the class of tests that inspired this whole idea, and I thought it
would really clean things up here. I don't know about "really," but it
certainly worked a little bit.

Working toward #329
Fix: Remove accidental to_csv() call in the WorkflowManager

I put this in for debugging purposes, and somehow forgot to remove it.
Removed TODO that had been done already
Fixed: Proper sorting in WorkflowManager._run_freq_agg()

By the looks of it, it was merely by chance that the results came out all
properly sorted in VIS 1. Now there's a much better chance they'l always
be properly sorted.

Closes #331
Refactored: Tests: the bwv2 tests for VIS 2

And they pass, too!

Related to #327