HW11 AI final submission:
(1). Finish the AI logic: simple implementation, heuristic version still
under development.
(2). Remove the dnd part which does not work under gwt envirnment.
(3). Test instruction:
	(3.1). Url: http://7-dot-abalone-game-smg.appspot.com/AbaloneGame.html
	(3.2). Use the GWT Emulator to test:
http://6.smg-gwt-emulator.appspot.com/GwtEmulator.html
	(3.3). Set "Network Delay" to be 500 or bigger than 500 to make the
AI's smoother.