Index instrument note ons and offs by channel rather than by channel program

[douglaseck]

I'm not 100% convinced this works. But it seems to work. Please give it a look.

[douglaseck]

fixes #84
Running the test code in the issue now yields:
[Instrument(program=0, is_drum=False, name=""), Instrument(program=0, is_drum=False, name="")]
[Note(start=0.022727, end=0.045455, pitch=43, velocity=20)]

I'm still not 100% sure this is a legit fix, but it makes sense to me.

[craffel]

This seems like the right thing to me. current_instrument is used to keep track of the most recent program change seen on each channel in a given track. This is useful for determining which Instrument to associate with each completed note. last_note_on, on the other hand, should be used for determining notes which appeared on a given track/channel/pitch prior to seeing a note-off. So, we still need both, but it makes sense (after 0ca39fd) to use event.channel as the key rather than current_instrument[event.channel]. I think this change was basically missing from 0ca39fd.

Of course, we need tests #32 for stuff like this.