Captioning support for VTT

Hi, 
This change lays the ground work for adding a source buffer for a text track for both browsers that support an do not support this. The out of band CC file is part of the MPD per the DASH spec. 

This change only parses vtt files and does not handle style and positioning for VTT at this time.  This may be a limitation of the TextTrackCue object at this time and more investigation needs to be done around this. 

Thanks 
- Dan