APU init and audio synthesis

APU init and audio synthesis

This lovely new lab shows how to set up the APU registers into a state
suitable for working with the NES audio subsystem.  It also introduces
some of the basics about how to play a sound out of the APU,
specifically the concepts of period, volume, length counters,
step-per-frame counter, etc.

Practically, the little guy now makes a rumbling, huffing noise as he
runs.  And I changed the animation cycle parameters to give him a little
hustle.