Create a new tile in a randomly selected empty 
        square.  The tile should be 2 90% of the time and
        4 10% of the time.

def new_tile(self, num=1):
        """
        Create a new tile in a randomly selected empty 
        square.  The tile should be 2 90% of the time and
        4 10% of the time.
        """
        for _ in range(num):                
            if random.random() > .5: 
                new_tile = self.pieces[0]
            else:
                new_tile = self.pieces[1]
            
            # check for game over
            blanks = self.count_blank_positions()
            
            if blanks == 0:
                print ("GAME OVER")
            else:
                res = self.find_random_blank_cell()
                row = res[0]
                col = res[1]
                self.set_tile(row, col, new_tile)