added stop and pause callbacks to AlsaSink

this uses my fork of pyalsaaudio which added PCM.stop to stop playback of
the device (https://github.com/stifal/pyalsaaudio)

I also implemented a Fallback:
    if PCM.stop is not available, simply close the device and open a new one

don't know if it is really necessary to use PCM.stop as the fallback should be good enough, but I implemented the fallback later.