fixed AlsaSink audio underuns on RasperryPI

- changed alsa device mode to PCM_NONBLOCK
    libspotify documentation says that the music_delivery callback
    should not block
- changed periodesize to an constant value
    setting periodesize to num_frames caused problems if num_frames got to low.
    for example the last music_delivery at the end of track
    raised an alsa error
- return in AlsaSink.music_delivery if num_frames is 0
AlsaSink: clear audio buffers on stop...

and audio discontinuity.
this uses my fork of pyalsaaudio which added PCM.stop to stop playback of
the device (https://github.com/stifal/pyalsaaudio)
implemented Fallback:
if PCM.stop is not available, simply close the device and open a new one
added pause command to the jukebox example
added start and pause callbacks to AlsaSink
fix memory leak in AlsaSink