Merge pull request #8 from proppy/master

frontend: add play button, play on every knob change
move main.go to backend dir
sensible default knob positions
hide main sound controls; move sound list
move play and share buttons a little
add musical scales to backend
add scale permute function; fix parseScale
wire up scale and perm knobs
fade out mp3 over 1 second
add Dockerfile and app.yaml for frontend