Change backend parser to mido

[craffel]

Resolves #99. This commit changes the backend from the python-midi package to mido. This change was tested by reading in 1,000 MIDI files with pretty_midi with both backends, writing them out, and then comparing md5 checksums of the written files. This tests both parsing and writing, which are the two things the backend is used for. The known differences are

• mido and python-midi parse some MIDI files differently mido/mido#62; the two backends do different things on a tiny fraction of arguably invalid MIDI files. This is arguably out of the scope of pretty_midi.
• MIDI files with corrupt pitch values are not handled correctly #34 is implicitly resolved by this PR because mido appropriately raises when there are invalid data "bytes".
• mido rightfully rounds microseconds per quarter note in its BPM calculations, midi doesn't. To maintain backwards compatibility, I am not using mido's functionality for this, but I will soon. This will break backwards compatibility (and change timing on the sub-microsecond scale).

@douglaseck, @cghawthorne can you run this code through your test suite to see if anything breaks before I merge and make a new release?

[cghawthorne]

In order to continue support loading midi files from file objects, this should be something like:

            if type(midi_file) in (str, unicode):
              midi_data = mido.MidiFile(filename=midi_file)
            else:
              midi_data = mido.MidiFile(file=midi_file)

Which is similar to what the previous midi package did:
https://github.com/vishnubob/python-midi/blob/master/src/fileio.py#L158

[craffel]

Great catch, should be fixed.

[cghawthorne]

Thanks, verified that all our tests pass now!