Mingus

Fixes some pending issues from the Python 3 migration, plus some other outstanding issues.

Changed
* Eliminate redundancy between Note 'dynamics' and 'channel & velocity'. **`dynamics` is now read-only**. `velocity` and `channel` can be set directly in the constructor.
* Safer MIDI event serialization with `int` bound checks
* Reorganize test suites and doctests

Fixed
* Documentation improvements (several commits)
* (58) Binary file open mode for MIDI and WAVE files
* (60) Wrong message sent in 'stop note container' event
* (64) MIDI input in Python 3, thanks to [Sam Gould](https://github.com/samg7b5)
* (4d82cbde) Standard pitch not honored in `Note.to_hertz()`

This is the first official release to support **Python 3** (≥ 3.5). I've tried not to break Python 2 compatibility, but I'm **officially not supporting Python 2.7** any further, especially since its end-of-life has come.

Some other changes — both old, that had been merged but not released, and new — are also part of this release:

Added
* (12) Add `channel` and `velocity` properties and setters to Note
* (18) Update LilyPond.from_Bar to handle minor, flat and sharp keys
* Better documentation for developers and standardization of the development environment

Changed
* (42) Fix: 3/x (3/8, 3/16, etc.) are not compound meters

Fixed
* (11) Documentation fixes
* (14) Fix LilyPond filename extension detection