Added alpha beta pruning to minimax algorithm.

One thing to take care of during alpha-beta pruning

Suppose that you have
root node with maxPossibleScore during tree evaluation.

                   (MaxScore given by 2)
1.(Some value less than MaxScore to be given by 2nd child of root)    2.MaxScore(this child has assigned alpha to be maxPossibleScore in the root after full evaluation) 3.(If the first leaf during DFS sets it to MaxScore, no further evaluations are made (as beta<=alpha). It is thus possible that it leads to sure failure.)
So only the first Max Possible Score's move should be chosen as the next move.
Other moves may or may not give max possible score and may even lead to failure.