rooting notes at C

refs #11; changing notes to be rooted at C rather than A, since that's what the rest of music does
correcting octaves relative to tonic

refs #11; notes are now rendered in their correct octave relative to the tonic or key; modes are still off, though
Merge branch 'issues/11-multi-octave'

pulling in changes because I'm observing some weird hanging behavior, possibly related to creating multiple audio contexts