Track DOM Implementation WIP 1

CC @humph

Few notes:
- Not much validation on TextTrack/TextTrackCueLists
- I forgot about adding addTextTrack() to HTMLMediaElement.. ><

I cut a patch against an up-to-date (as in today, Wednesday February 13th 2013 at around 11PM) copy of m-c. Going to post the patch on bugzilla tomorrow. Here it is: https://gist.github.com/daliuss/4950639