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.
You should have two empty lines here. This is one of the two style issues catched by the flake8 test at Travis. You can install the flake8 tool from PyPI and run `flake8` in the top level pyspotify dir to check for issues like this locally.