Added Alpha-Beta Pruning

Now the minimax search is optimized via alpha-beta pruning technique.
Therefore it's much faster and the minimax search goes on depth 3
currently (the time required for each move is ~27 seconds). Also
instead of considering all 21 possibilities of dice each time, we use
14 precomputed random dices. It gives a better performance in terms of
time and is also a good estimation.