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.

We'd need to do that on WSS side as well if we do. Since WSS supports message by POST it has the same async issue. Logic would be on both GAE / WSS to place offer in front of candidates whenever delivering batches of messages.

However, there would still be need for a queue on client side / need to wait for offer. e.g. scenario:
C1 posts offer + candidates to GAE. 
C1 candidate POSTs complete.
C2 registers, receives C1 candidates.
C1 offer post completes, gets forwarded to WSS.
C2 receives C1 offer.