Reduced 'Pop' sound on playback/stop

- Created improved ramp to smooth transition between last value of OCRnA
and new value in between stop/start of playback. Intial setup will still
cause a pop sound.
- Modified stopPlayback code slightly
Merge branch 'master' of https://github.com/TMRh20/TMRpcm

Conflicts:
	TMRpcm/TMRpcm.cpp