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

should also fix issue #83