Prepared abjad-ipy-midi for integration into Abjad mainline

I've refactored the abjad_midi extension in preparation for integrating it into Abjad's mainline repository.

The changes are largely stylistic - rephrasing docstrings, renaming variables for clarity, adjusting whitespace. 

The largest change is in refactoring the entire module into a callable class called `Play`, an instance of which (named lowercase `play`) replaces Abjad's top-level `play()` function. This allowed me to dispense with the few uses of the `global` keyword. Those few module-level global variables could be rewritten as instance variables on our singleton `play`, which makes the code a bit simpler.

I've tested this with Python 2.7.6 and IPython notebook under OSX 10.9, but not yet under any other Python version or operating system.