Move session/tunnel to webrtc/libjingle.  This is part of the ongoing effort to move Jingle-specific things out of WebRTC and into its own repository.   I won't submit this until all other projects have moved off of compiling this as well.

R=juberti@webrtc.org, pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/38369004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7953 4adac7df-926f-26a2-2b94-8c16560cd09d