XHTML fixes

This branch contains two fixes for the XHTML code. One fix, which is also required for DocBook, automatically converts HTML 5 named entities (e.g. \&sect;) into numeric entities (\&#176;), since XML has only five named entities. The other fix adds a namespace to XHTML elements.

With these two fixes, I can now convert the user manual (in the asciidoctor.org repository) into XHTML and make it render correctly in Firefox.

I realize the large table of entities is ugly, but if you have a better way to handle them, I'm all ears.

This addresses issue #904.