Support for subtitles

[saerdnaer]

Beginning with 30C3 we have subtitles in srt format for some talks. The player should display this subtitles and allow to switch between different versions.

We should maybe also provide links to add new or improve existing subtitles at amara.org
See also:
http://subtitles.media.ccc.de/
http://subtitles.media.ccc.de/data.json