Minor Formatting Updates for IntervalNGram

Mostly to use unicode strings where possible.
Updated importer.import_piece() and Its Tests

- Added Sanctus.krn to the test_corpus, to test importing Opus objects
- Changed the try/except block to only catch converter errors
Some Initial Tests for the Experimenter Module

- IntervalsLists has some decent stuff
- IntervalsStatistics has some barely-acceptable stuff
- I also revised the documentation for IntervalsLists.__init__()