Improved tracking of device state (stopped, playing, or paused).
Also, tied starting and stopping the AudioUnit more directly to the
state.  No need to change the state when preparing or unpreparing
wavehdrs.  Pausing overrides both playing and stopped states; if
stopped, pausing prevents output from starting when the program
writes.  When, restarting from the paused state, the device starts
playing if there are queued wavehdrs.  Otherwise, it goes to stopped
state.