add 'reason' attribute to MSRPChatDidFail notification

darcs-hash:20090312100838-b2873-c7d367b4a2e99fb4bfa203812068a858ecc08abf
allow customizing Session class in SessionManager

darcs-hash:20090312100908-b2873-692eacc32c615cf08959206bbd2b61b284450ae9