cpim.py: fix MessageCPIM's __repr__

Ignore-this: 5ae6e3b53ff606b452ddebd48b425df7

darcs-hash:20090203102242-b2873-ee40a34b29edbfde5bc25d435e2089b8bfac17fb
add DateTime header to outgoing messages and use it as screen date for incoming messages

Ignore-this: ed14c3176dc824e0b1f225f8accb61ed

darcs-hash:20090203102345-b2873-9ec5ed82a99248439996835b3b8225a19cbe6b87