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.

I think it's possible to switch to just using transactions instead of using a lock, but we'd need to add exception handling code for when transaction fails. Transactions will likely be more performant because they'll operate on same entity group (clients in the same room in this case). Probably a later CL.