setLocalDescription fix

According to http://www.w3.org/TR/webrtc/#dom-peerconnection-setlocaldescription.
successCallback of setLocalDescription is not nullable.
This patch also fix problem in firefox 24.
Bugfix. Permit receiving a 200 OK to a INVITE before any 1XX provisional

- Set the RTCSession status to CONFIRMED before sending the ACK
Fix #119. Set the optional 'DtlsSrtpKeyAgreement' constraint to *true* for incoming RTCSessions
Fix #107. Stop spamming provisional responses