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.

Yes, the messages cached on GAE can be out of order when queried from the datastore.