Added a default constructor to the Ball class.

The constructor sets the Ball id to be 0 by default.
Added a new function to the Server ABC.

getPaddleFromID and getBallFromID are both implemented in the concrete
server implementation as well.
Added two new tests which uses the new getPaddleFromID function.

One asserts the fact that the server listens to its paddle destination
requests. The other makes sures that the paddle id returned by
Server::makePaddle is registered in the Paddle::id field of the
corresponding paddle.