* All wt2wt, html2wt, and all but one html2html tests pass in
hybrid mode when entire html2wt code is run in PHP
Set "Serializer: true" in the html2wt section of phpconfig.yaml
* The single failing html2html test is a <gallery> test which is
presumably related to the unported <gallery> extension code, but
not sure. Not investigating it now.
* Update Parsoid Extension API to provide access to extension source
without exposing internals.
Change-Id: I6d6e21ad2324acfc4306b32c9055d6c088708c48