This commit adds one new property, which is the URI that mopidy is using as the source to play the media file. The significance here is that if you use something like Podgrab to auto-download and push podcasts to a directory in Mopidy's media files, and if it has 'podcast' in the name we can use that instead of the hit-or-miss check on the MP3 genre of Podcast, which is a convention not many podcasts use.