gajim: Add support for LaTeX markup.

This needed a bit of patching the paths to the latex and dvipng executables.

In addition, we also address a small bug by using sed:

tmpfd, tmppng = mkstemp(prefix='gajim_tex', suffix='.png')
tmpfd.close()

This obviously won't work, because mkstemp() actually returns a file descriptor
_number_ and not a Python file object.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>