Updated EngineBuffer class to new Invitation API for MSRP scripts

darcs-hash:20090123171029-58103-56f4b019decae91165f067a163e2e4e114a7e111
Updated sip_audio_session.py to new Invation API

darcs-hash:20090123171113-58103-7ef2e0f45475fb727c0225827a4e0f349764560d