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