Added rhythmmakertools/gallery directory.

The directory currently contains one PDF:

    rhythmmakertools/gallery/EvenRunRhythmMaker.py

Implemented a private _gallery_input class variable on EvenRunRhythmMaker:

    ### GALLERY INPUT ###

    _gallery_input = (
        ({
            'denominator_multiplier_exponent': 0,
            'beam_each_cell': True,
            'beam_cells_together': False,
            },
            [(4, 8), (3, 4), (2, 4), (1, 16), (1, 16), (7, 8), (5, 16)],
            ),
        ({
            'denominator_multiplier_exponent': 1,
            'beam_each_cell': True,
            'beam_cells_together': False,
            },
            [(4, 8), (3, 4), (2, 4), (1, 16), (1, 16), (7, 8), (5, 16)],
            ),
        ({
            'denominator_multiplier_exponent': 2,
            'beam_each_cell': True,
            'beam_cells_together': False,
            },
            [(4, 8), (3, 4), (2, 4), (1, 16), (1, 16), (7, 8), (5, 16)],
            ),
        )

Implement new static method on EvenRunRhythmMaker:

    EvenRunRhythmMaker._write_gallery_to_disk()

The methods reads the class's gallery input, creates a LilyPond file object,
populates the LilyPond file with one score block per input block, and persists
the resulting PDF to the new rhythmmakertools/gallery directory.

The pattern is not yet refined; following commits will beautify output and
streamline file creation.

NOTE: following commits will mildly refactor the lilypondfiletools classes.

NOTE: following commits may add or change a keyword or two in PersistenceAgent
methods.

MODEL: galleries are basically fancy test batteries. Future commits will
persist gallery content as (testable) .ly files and allow py.test to check the
continued validity of gallery output, just as in musicexpressiontools in the
experimental branch. Such test affordances will be implemented after the
generate and display traits of gallery manipulation are worked out.