Subtitle Support

Hi Joern,
tagesschau.de has been providing subtitles (in ttml format) for "full" tagesschau broadcasts for a while. 
I've added functionality to download and convert subtitles to SRT and include them for playback.

The current code to add the subtitle on the list item has only been tested with Gotham and will need to be adjusted for future releases as per this discussion: http://forum.xbmc.org/showthread.php?tid=154805&page=7&highlight=subtitle

Maybe you find some time to merge the changes and release a new version.

Thanks
--Henning