update game for alpha/beta pruning

added alpha/beta pruning to prevent the AI from going down every tree
that may not result in a win

added depth purning to preven the AI from thinking too many turns ahead
since it isn't quite necessary

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* Also updated determine-score to remove the 'opponent' scoring since my
algorithm alternates and in turn never actually detects an opponent win
Merge pull request #9 from zachmokahn/alpha-beta

update game for alpha/beta pruning