add "midi" sequencing support

[kn0ll]

i'd like to add what would essentially act as midi inputs and outputs for AudioletNodes (actually, perhaps AudioletGroups only in this case?). my ultimate goal is midi effects, like arpeggiation. a nice side effect of doing this through a midi chain (similar to Ableton) is that we could eventually incorporate true midi i/o into the system with minimal effort.

i have this working as proof-of-concept outside of audiolet you can see here... i ripped out the relevant code to help illustrate here: midi interface proof of concept arp user-keyboard-to-midi message

your thoughts?