Tic Tac Toe

We are going to implement a two-player Tic-Tac-Toe game similar to (but simpler than) http://playtictactoe.org/.

Tic-Tac-Toe is a paper-and-pencil game for two players, X and O, who take turns marking the spaces in a n x n grid (usually 3 x 3). The player who succeeds in placing n of their marks in a horizontal, vertical, or diagonal row wins the game.