Added message to sip_register for registration refreshes

Ignore-this: 8c35e9e44b25f8d40e2cc811c7354163

darcs-hash:20090408091127-1e3d8-7c68be657a27fd98b3d5f8ae0f59cbaf493aa63e
sip_session: add :echo command

- examples: :echo (print current), :echo + (inc by 10), :echo 150 (set to 150), :echo -20 (dec by 20)

darcs-hash:20090408092943-b2873-18dcc6fe571f0bfc2130972164556e355786d195