Add support for websocket reconnect and sending BYE to apprtc

Major changes:
1. Do not remove the client when the websocket is closed, instead start a timer and remove the client only if it does not reconnect within the timeout.
2. When removing a client, send a POST to https://apprtc.appspot.com/bye/roomid/clientid to help GAE clean up the room. 

Minor changes:
Moved the constants to collider.go for easier management.
Renamed some var to shorter names.