Add support for pedal

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.