commit -- several bug fixes to the LaTeX exporter

[Joaqu�n Cuenca Abela]

I've fixed the bugs that Pierre has logged relatives to the LaTeX
exporter.  Now, a document with columns can be exported to LaTeX and
exported file can be processed by LaTeX (but like a document with just
one column).

Now the exporter ignores all the characters above 0xff (if anybody knows
how LaTeX can use correctly these characters, please contact me).

In addition, I've added superscript/subscript capabilities.

Thanks to Pierre for the greats bug reports!

--
Joaqu�n Cuenca Abela
e-mail: [EMAIL PROTECTED]