Let VTTCue have a direct region member instead of a regionId string

https://www.w3.org/Bugs/Public/show_bug.cgi?id=24380
Remove the VTTRegion.track and .id attributes

https://www.w3.org/Bugs/Public/show_bug.cgi?id=24380

There are still uses of the region identifier concept to clean up,
in the parser and in getRegionById().
Remove the extensions to the TextTrack interface

https://www.w3.org/Bugs/Public/show_bug.cgi?id=24380

With a direct association between cues and regions, the text track
itself doesn't need to know about regions, simplifying the API.
Merge pull request #31 from foolip/regions-api

Don't extend TextTrack with WebVTT-specific attributes