FIX: update outputlilypond
FIX: add 'Staff.shortInstrumentName' for PartNotesIndexer

Almost finished with #276
FIX: Annotated Scores Have Part Names

Closes #276
FIX: update outputlilypond yet again
Update VERSION_HISTORY for 1.2.0
FIX: update version to 1.2.1
FIX: temporarily disable part names, since they may be incorrect

Refer to issue #301
FIX: Rename Const Error Message to all Capitals

Because why not?
FIX: WorkflowManager._intervs() Complains with Invalid Voice Pair

As per issue #303, the "intervals" experiment can only deal with pairs
of voices. Now we complain when we get invalid voice combos.

NB: I didn't change the API docs because I think this is covered in the
existing description.

Closes #303
FIX: update outputlilypond
Fixed: outputlilypond Submodule Uses Public URL
Fixed: Update 'requirements.txt' to Allow music21 1.7.0.

1.7.1 is better, but not available from the PyPI, so it complicates
installation. This will help Travis-CI.
Test: Add .travis.yml File for Travis-CI
Fixed: re-update 'outputlilypond' path
New: Add Travis-CI Build Status Image
Merge branch 'master' into devel_2