the export command (do_export in admin.py) should allow
to specify classes which should /not/ be exported; this
way it would be possible to backup e.g. the msg and
file topics incrementally (some jokesters like to send
screenshots as BMP files (or worse) to our tracker, and
having those in every single backup is not very desirable).

My simple patch excludes the given classes when the
specification starts with a '-' sign (sorry, I'm
offline here relating to CVS, so the diff format might
be slighly inaccurate).