Docs: Add the LilyPond Experimenters to the API

Though of course I wrote the docstrings, the analyzers.experimenters.lilypond
module wasn't being built. Now it is.

Related to #330
Fixed: LilyPondExperimenter.required_score_type = 'stream.Score'

As this class changed from an Indexer to an Experimenter, I lost the
functionality of automatically importing the Score for outputlilypond. This
doesn't require a change in IndexedPiece---just in the Experimenter.

I also added a note in the Experimenter and TemplateExperimenter, saying it's
possible to set required_score_type.

Related to #330
Style: improved in IndexedPiece

Mostly, I changed all the strings to use format(), all the exceptions use hidden
class-level static strings, and the unicode things are just strings.
Test: the previously-untested branch of IndexedPiece.get_data()
Fixed: LilyPond output in the WorkflowManager

The WorkflowManager._make_lilypond() method was not yet updated for VIS 2. Now
it is.

Closes #301
Closes #330