added timeouts for msrp connections, proper closing for pypjua objects (unregister, invite.end)

* changed EngineBuffer to allow central shutdown of all pypjua object; made sip_msrp_client use it
* added timeouts for MSRP connect/listen
* better logging for pypjua events: prepended with filename:lineno
* made InvitationBuffer.accept to wait for ESTABLISHED event
* sip_msrp_client.py: replaced SystemExit with GreenletExit
* sip_msrp_client.py: removed log_events function that could eat useful events
* moved accepting code to msrp_accept

darcs-hash:20081113120314-b2873-64c15798965a3121c85cfe3744e09842b1eb9530
send 488 response to invite if msrp cannot be connect/authed etc

darcs-hash:20081114065803-b2873-b8b9cee553dec30ca688b0b2928716934a7a6ed0