Some cosmetic improvements to center the score, change paddle color and draw target like a line-ball.

Andrea
Move autoplay on a (more) event based mechanism.
This to hide internals of the autoplay algorithm and replace strategy.

There is one love2d issue where tabls cannot be passed as arguments of events
(so we pass the key "player_1" or "player_2" as a lookup in game, rather than
the players themselves).

Andrea