Update PBS Kids plugin to support Gotham and have more shows.

* Update xbmc.python dependency so addon is not broken in Gotham
* Shows list is now dynamic instead of hardcoded so the latest list of
  shows will be fetched from the PBS API.
* Update to support new JSON API for PBS kids.
* Refactored and cleaned up some of the code.

Fixes #2
Merge pull request #3 from conorbranagan/conor/pbs-kids-gotham

Update PBS Kids plugin to support Gotham and have more shows.