Mopidy 0.18 updates

Update to use new extension API and new backend class locations as the old ones will be removed in the near future. While I was at it, I switched to use loggers named after the `mopidy_somafm` Python package instead of loggers associated with the core Mopidy project and fixed all flake8 warnings. I also did some other minor tweaks to the readme just to keep it sync with other extensions.