update game for depth 3

maximum depth reached is 3, added a function to detect if the game state
is 'winnable' and assigns value to that
update AI for depth-pruning
tweek depth pruning

updated depth pruning to go to a depth of 2, calculates a value for a
board that doesn't have a winner based on possible winning positions and
potential future winning positions
Merge pull request #11 from zachmokahn/depth-pruning

Depth pruning