Refactor: a better way of initializing the empty positions
Refactor: remove explicit reference to the class
Refactor: change how the bounds of the array are checked
Refactor: rename to remove references to player in the board class
Refactor: remove explicit reference to class
Refactor: change how the default value for @board is set
Refactor: remove NO_WINNER constant since it's not needed
Refactor: change how the winner is checked