added tic tac toe game to play against computer

Computer is still not smart, only playing based on randomization. Known
issues related to logic of when winner is established, including (1)
when player wins with the last move on the last unoccupied box, (2)
when player wins before all boxes are occupied and computer continues
to make next move, and (3) when winner is established user can continue
to click/play on unoccupied boxes