Wrap the cue in an anonymous div with inline positioning.

This functions as the cue background box and allows us to only
have the text have the background color. As well as allowing us
to use getClientRects() to get each line of text as a TextRectangle
which will allow us to get the exact line heights.
Calculate the line height exactly.

We can now use 'getClientRects' to get the exact line height
of any of the lines in the cue box.
Update spec compliance part of REAME.
Fix the VTT DOM shim test name.
Release v0.8.0
VTTCue should hold a reference to its VTTRegion.

Updates to the spec have the VTTCue holding a direct reference to
its VTTRegion instead of the VTTRegion's ID. VTTRegion::Id also
no longer exists.