Remove midi dependency

[craffel]

Make pretty_midi able to do the MIDI data parsing, too

[carlthome]

Yes, please. python-midi lacks Python 3 support on PyPI (though there is a dev branch with Python 3 support on their GitHub repo) making setup.py installs complicated.

[Conchylicultor]

Or use mido for the low level parsing. The development is more active than python-midi and at least it's python 3 compatible.

[craffel]

This was resolved in #101, though this issue is more focused on creating a pretty-midi-native parser. I don't think there's a strong reason for this except speed, which is addressed in #102.