Refactored SDP handling

Ignore-this: 2aaaca3853a4873f6196e66ecf6fc6fb

Each RTP transport will now have the view of the whole SDP plus the
index to which that stream belongs. This allows us to build subsequent
SDPs having PJSIP attach the transport specific information.

This finally allowed us to generate a re-INVITE after ICE negotiation was
done, in order to inform the other endpoint (and any middleboxes) about the
negotiation result.

darcs-hash:20140520124630-56b4d-a70881dbca9d27c5267146c2e5624711161a0130
Raised core revision

Ignore-this: a77b17c5811674bd79ee7a5300c6cb11

darcs-hash:20140520124913-56b4d-84c66daa083503070a42dfa56cce14b5c733fa45