* Added Best Score, Clock and Pause

... when the score is greater than the former best score.

* Added clock to show elapsed time. 
* Added pause control (key P) and white overlay to nearly cover the tiles, showing two buttons (Keep Going and Try Again) to resume the game or start a new one. 

Timer stops during pause or when 2048 tile is reached. It goes on after Keep Going button is clicked. It restarts if Try Again button is clicked.