Stalemates are not determined

[TTitcombe]

The game does not calculate if a stalemate has occurred. This can lead to infinite play.

To replicate:
Play a random bot against a random bot, using the following script

from alphadraughts import Game, RandomBot

game = Game(white=RandomBot(), black=RandomBot())
game.play()

There may be a breakthrough, but most of the time the game will continue indefinitely (stalemates occur frequently because of #23 - pieces block the middle which leaves the remaining pieces and kings to wander about aimlessly)