Split up (Jingle)Session from BaseSession.  This is part of an ongoing effort to move Jingle-specific code out of WebRTC and into its own repository.

This is an un-revert of r7992 and r7993.

R=henrike@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/32869004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7939 4adac7df-926f-26a2-2b94-8c16560cd09d