Cleaned up stream/timespans.py.
Added new stream/timespanAnalysis.py module.

The module will house logic related to performing voice-leading analysis with
timespans.

The module name and location are provisional.
Updated stream/__init__.py to import timespanAnalysis.
Migrated Horizontality from timespans to timespanAnalysis.
Migrated Verticality from timespans to timespanAnalysis.
Migrated VerticalitySequence from timespans to timespanAnalysis.