Detect draws

[cropleyb]

Draws are very rare in Pente, and are not currently supported. A simple check would probably be enough: move number - 2 * total pairs taken >= board size * board size A game may be drawn slightly before this (when the board is almost full) but detecting this would be much more complicated, and very rarely used. A draw should have it's own sound effect (1 should be enough!), visual representation (flag halfway?) and internal representation (Game.won_by() == P1 + P2)

[cropleyb]

This is probably implemented now - needs testing.