With this commit, we stop waiting for the websocket connection to be
established before the game starts.
Now, the Connection type is responsible for waiting for the websocket
connection before writing.
In order to simplify special moves like en passant or castling for the
client, we want to deliver the board state after every move (and not
only start square and end square).
With PGN we can encode a chess position into a string.
This commit implies changes to logic of the pieces' shortnames. This
will break the client/server connection (at least for promotions).
