Add a test for cue IDs.
Initial implementation of the processing model.

This patch implements support for processing multiple VTTCues
from multiple TextTracks at once as well as applying the initial
CSS positioning and styles to the VTTCues once they are converted
to a DOM tree.
Wrap vtt.js in a function expression.

Also export WebVTTParser on the global whether we
are running in node or otherwise. This was suggested
by Dave on bug 895091 to take care of the node,
web page, and browser use cases.
Bulk README.md updates.

Documentation for processCues(), VTTRegion bundled in the
web page polyfill dist, and updates to the JSON test file
example.
Make cue div CSS values 'vh' and 'vw' CSS units.

The specification has left and width as 'vw' CSS units
and top and height as 'vh' CSS units.