libpqxx has HTML docs

Thanks to Bruce's help, libpqxx now finally has some HTML documentation.
Converting this from SGML is still awkward, since I don't have the tools
to do this myself. Still, let me know if you find anything wrong.

There are still some errors in the SGML documentation; any help fixing
this would be most welcome. The goal is to eventually integrate this
stuff into the postgres tree, so it'll have to compile to decent HTML.

John Anderson has worked on autoconf / automake files, so once this is
integrated, configuration for a particular system should also be a lot
less tedious as well.

Jeroen

PS - my habitual plug: libpqxx is a C++ frontend API to PostgreSQL, ie.
an alternative to the old libpq++. I hope it will replace the latter
as the standard C++ API. Find the source code at:

http://members.ams.chello.nl/j.vermeulen31/proj-libpqxx.html