Added property to show tvshow/season progress in %

Adds a new property "percentplayed" for tvshows and seasons which returns the amount of played episodes in percent.
also useful to display a progress image, here´s an example of what we skinners can do with it:
http://i.imgur.com/cx43M6L.jpg (tvshow level)
http://i.imgur.com/f8J7V5X.jpg (season level)

if you know a more elegant way to cast the data types let me know :)
same goes for the label, not sure if "percentplayed" is the best choice.