Add ve.createDocumentFromHTML()

Converts an HTML string to a brand new document using an iframe hack
(proper ways to do this exist, but don't work cross-browser).

Parsoid will serve us full HTML documents rather than document fragments
soon, so we'll need this functionality (along with some other changes
that I'm working on now) to deal with that change.

This doesn't currently work quite right in IE8 (although we have lots of
other issues in IE8) as well. It's not that hard to fix up though: we
just have to leave the iframe attached to the main document (should
probably provide a destroy function in that case) and the tests have to
deal with the fact that IE normalizes <head></head> to
<head><title></title></head> .

(For backwards compatibility, we'll have to deal with document fragments
as well; this will be implemented as an MW-specific hack in the
integration in the next commit.)

Change-Id: I15f877583c39124ba1c5e8e22585297ff3bac8d6