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: made PCM mode confugurable agian
AlsaSink: made periodsize configurable, fixed typo
added remove_playlist function
change to conform
changed redundancy
Merge pull request #84 from stifal/fix-alsa-underruns

fixed AlsaSink audio underuns on RasperryPI
alsa: Style tweaks
Update changelog
Update AUTHORS
changed remove playlist function and added a sleep
added pause command to the jukebox example
AlsaSink: clear audio buffers on stop...

and audio discontinuity.
AlsaSink: added start and pause implementations

Conflicts:
	spotify/audiosink/alsa.py
AlsaSink: fixed memory leak

Conflicts:
	spotify/audiosink/alsa.py
AlsaSink: fixed ALSAAudioError under some conditons

bug was caused under this condition (jukebox.py):
    * play a track
    * pause
    * session.seek(0) # in the jukebox shell
    * play
    * pause
    * play
    * raises: "ALSAAudioError: File descriptor in bad state"
AlsaSink: added testcase for bug in last commit
AlsaSink: made testcase work on Travis
documented remove_playlist method
Check the playlist index is within range before removing