Remove region processing.

We're not planning to support it right now and the region and
cue processing algorithms will be unified in the near future
so there's no point to keep this part of the algorithm. Removing
it makes the code clearer.
Merge CueBoundingBox and BasicBoundingBox.

There's going to be one code path for Regions and Cues in the
spec so there's no point to have an inheritance scheme that
allows for multiple kinds of boxes.
Update README to reflect removal of support for region processing.