Rewrite the engine to make it work without making it into an observable
Fix bug: Calling all_moves with the incorrect number of arguments in Novice and AdvancedBeginner
Add a to_s method for displaying a friendly version of a grid
Keep track of the last event that occurred within the engine
When the game is squashed, make it so that it is the other player's turn to play
Build a command-line Tic-tac-toe client