Add user rating to videos

Figured I'd have a play with the XBMC code, wanted to add my own rating
to videos in XBMC for a long time, and eventually have it sync up to
imdb, trakt, moviedb and tvdb.

Includes a simple "Rate ... " context menu.

Based the work on https://github.com/xbmc/xbmc/pull/607 as a starting
point