Revised Metadata Import & Docstring for add_index()

The music21 part names and title metadata fields are too finnicky to use
directly, so we should use these wrappers.
Add FrequencyExperimenter and Minimal Tests

The FrequencyExperimenter calculates the number of occurrences of tokens
within a previously-calculated index.
Improvements to FrequencyAnalyzer

Mostly that its experimenter_func() uses fewer loops.
Added Optional Dependencies for pandas in Engineering Docs
Additional Instruction in TemplateIndexer

Just says that you can remove a line from __init__() in some cases.
Merge branch 'visX-devel' of github.com:crantila/vis into visX-devel
Merge branch 'visX-devel' of github.com:crantila/vis into visX-devel
Delete Unused Files

We won't need these anymore. If we want to use them in the future, we can
use the magic of version control to look at them!
Adjust */ __init__.py and run_tests.py for Previously-Deleted Files