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 the 3rd try)

R=juberti@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/29309004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7956 4adac7df-926f-26a2-2b94-8c16560cd09d