sip_session2: add inbound/outbound ringing and tones

darcs-hash:20090429155348-b2873-f14da4059ba3984af9640accbad490b7d3301e55
desktopstream: raise SDPNegotiationError instead of AttributeError

darcs-hash:20090429155646-b2873-43c85566b15cdba4fdc6c544fe8888dc1f955358