Add support for pedal

As scherzo listens to the pedal, it will analyze the chord harmony based on
notes that continue to sustain due to the pedal.

I'm not 100% comfortable with the empirically-derived value of 64 for
the sustain pedal. I'd feel a bit better if I had found a symbolic
constant for this in the ALSA header files. As things stand now, I'm
not sure if the sustain pedal on another keyboard will return the same
value.