Document repeating tracks.
Add a level to the library so repeating track works better with Mopidy-MusicBox-Webclient.

Also carry forward all station data we need in the url.
Merge branch 'master' of https://github.com/rectalogic/mopidy-pandora

# By Andrew Wason
# Via Andrew Wason
* 'master' of https://github.com/rectalogic/mopidy-pandora:
  Add a level to the library so repeating track works better with Mopidy-MusicBox-Webclient.
  Document repeating tracks.