Update Balancer to latest HTML5 spec

Update Balancer to latest HTML5 spec

This corresponds to the 1.0.27 release of domino, and matches the
latest HTML5 spec as of 2016-10-18.

Changes include:
* <menuitem> is no longer an empty element.
* <isindex> has been removed.
* Updated html5lib-tests (copied from domino 1.0.27).
* Round-trip-safe serialization of <pre>/<listing>/<textarea> is only
  used when "tidy compatibility" mode is enabled; the behavior in
  the HTML5 spec no longer cleanly round trips.

Change-Id: I656944b0d7bb6c3c0e4fe44fc6ebd1a4c36412ad