Bug 895091 - Remove references to C WebVTT parser and unused code r=rillian

- Remove left over references to WebVTT C parser in TextTrackCue and
WebVTTListener classes.
- Remove miscellaneous unused code as well and included libraries as
well.
Bug 895091 - Add WebVTT parser and wrapper IDLs r=rillian, ted, gwagner, khuey

- Added vtt.js, a JS WebVTT parser, to Gecko.
- Added a wrapper to interface between it and Gecko.
- Updated TextTrackCue and WebVTTListener to work with it.

--HG--
rename : content/media/WebVTTLoadListener.cpp => content/media/WebVTTListener.cpp
rename : content/media/WebVTTLoadListener.h => content/media/WebVTTListener.h
Bug 895091 - Add JS WebVTT parser code in (vtt.js) r=rillian, ted, gwagner, khuey

- Current source code for vtt.js can be found at:
https://github.com/andreasgal/vtt.js
- vtt.js code taken from commit 6bca57d265fc58cff091148b426ec69f576057d7.
Bug 895091 - Turn text track cue tests back on r=rillian
Bug 895091 - Test for multiple WebVTT files loading at once r=khuey