The message processing lets plugins manipulate incoming and
outgoing IQs, presences, and messages.  I imagine it being
an extension of GSD similar to the IQ system.  If GSD were
general enough (issue44), this system would be very simple
and possibly more efficient.  issue44 would be very nice to
solve for several reasons, so perhaps it should be a dependency.

Applications of the message processing system:
- spellcheck autocorrector
- auto-linkifier
- Fast Presences would get better behavior (recv a message,
output a presence)