Now write_midi excepts a list of instruments to use for each track written to the MIDI file, and each track gets its own MIDI channel.
Merge branch 'master' of https://github.com/jtauber/sebastian

Conflicts:
	sebastian/midi/write_midi.py
Merge pull request #19 from pmargani/master

write_midi now maps each track to its own channel, and instruments can be specified.