add randomization to same-score move selection

[danlangford]

from #32 @pappde notes the following

right now if two moves have the same "score", it picks the first one.

  	if (t.score[i] > t.best_score)
  		t.SetBestMove(attack, t.score[i]);

You could add some randomization to select from multiple moves that have the same score (which may be 0).

[pappde]

This is low priority, since it will typically only occur in edge cases where it probably doesn't matter anyways.

Currently, there is bias for the first move. There are two approaches that could be taken to reduce the bias:
A) low-cost add a simple tweakable probability (e.g. a 50% rand check). This would be "fair" if there were two moves, and otherwise would bias the later moves
B) build a list of moves that match the score, then randomly select from that list