Merge apprtc-collider branch to Master

The apprtc-collider branch contains the changes to 
1. Make AppRTC use Collider for signaling
2. The user joins the room on a separate POST message to GAE, instead of on the first GET
3. The first user of a room is the caller and GAE caches his signalling messages, which are pushed to the second user when the second user joins the room.

Out of order on datastore because clients POST multiple async requests to GAE simultaneously, and their order of completion is different from order of request.