Fix PortAudio underruns (#132) by using async playback

This switches to async playback and fixes the periodic underruns observed in issue #132. Suggestions on style are welcomed.
I try to follow [Google's Python style guide wrt import ordering](http://google-styleguide.googlecode.com/svn/trunk/pyguide.html?showone=Imports_formatting#Imports_formatting), so this would become:

``` python
from collections import deque
import logging
import sys

import spotify
```