AppRTC: Use WebSockets instead of Channel API.

With this change no messages are stored on GAE. Instead, they are stored on Collider.
GAE still keeps track of which users are connected to a given room.
CR comments. Loopback case now handled.
Delete roomid/clientid on WSS in case registration failed.
Use websocket for send if registration has occurred otherwise just POST.
CR comments.

Moved loopback code to its own file, and only serve it when needed.
Log websocket and websocket server errors.
Merge pull request #216 from tkchin/apprtc_collider

AppRTC: Use WebSockets instead of Channel API.