Most of the time, sending a disconnect event to one of the players is enough (the player will close the connection
which will be shut for the other player).
However! In the rare case where the WebRTC connection is not yet established, if we close the connection on one of the player,
the other player will try connecting until a timeout happens (during this time, the circle with the name is displayed for nothing).
So now, we send disconnection event to every body (not only the people in the group, but also to the person leaving the group)
Socket.io can stringify JSON messages itself, so there is no need to pass a string to "emit". You can pass a serializable object!
This commit removes all the useless toJson() methods, JSON.serialize and JSON.parse!
Woot!
