Ported spotify-notify.py to Python3

There were some small changes necessary to make this script Python3 compatible. This version of spotify-notify doesn't work with Python2 anymore. However, I hope the changes were small enough to not have added any new bugs. I tested this on Ubuntu 14.04 TLS, where it works great.

The reason for this port is that Python2 is more or less deprecated and Ubuntu is trying to ban Python2 from its default installation. They could not hit this goal in Ubuntu 14.04 LTS but they are certainly going to remove Python2 at some point in the future.

You could consider tagging the old version with 2.0 and this version with 3.0 or whatever. Then one could mention in the README, that version 2.0 is the last version to support python2 and that all newer releases require python3 to work.