Add UI features for single player mode.

* Upon starting a new Game, user should be prompted for Single Player or Two Player Game.
* If Single Player Game, ask user if they want 1st Move or 2nd Move.
Add a real basic AI for single player mode.

* Computer opponent will always make a valid move.
* Computer opponent's move can be totally random so long as it's valid, but will always make a winning move if there is one available.
* Bonus Points if your computer AI can beat 2 out of 3 five year olds (I hope it can beat)