fixes #18, keep track of which instrument/note pair is playing on each audiotag 'channel', then on noteOff/chordOff find the right audiotag 'channel' to pause().
when stopping a note, look through the channels from oldest to newest, stop only the first matching note, then null it out and return
need to use nid to find channel to pause, and forgot to add i to channel_nid to get nid
Merge pull request #20 from bsstoner/audiotag-pause

fixes #18, keep track of which instrument/note pair is playing on each a...