Migrate module_spotify.py to use new API and add tests

Spotify has [deprecated](https://developer.spotify.com/technologies/metadata-api/) their Metadata API
  * Updates `module_spotify.py` to use the new [Web API](https://developer.spotify.com/web-api/)
  * Tells genre for artist if specified (example: `[Spotify] M.A. Numminen (Genre: classic finnish rock)`
  * Adds tests for module.