WIP-1/5: New: ngram.NGramIndexer._make_column_label()

Part of the NGramIndexer's slow but inevitable journey toward VIS 2.

Part of #278
WIP-2/5: Better standards-following in test_ngram.py

Part of #278
WIP-3/5: Refactor: NGramIndexer class docstring, and __init__()

I also wrote tests specifically for __init__(), and revised all the
descriptions of the settings the indexer will need.

Related to #278
WIP-4/5: Remove unnecessary u'' literals in NGramIndexer._format_thing()

Related to #278
WIP-5/5: Refactor: NGramIndexer support VIS-2-style DataFrame

With the past five commits, the NGramIndexer now supports input of
and output to the VIS-2-style DataFrame format.

Closes #278