add boardgame class. test unit. tests for constructing board, checking positions, setting positions.
add check position to refuse moves on played positions.
add simple method to check if a position is empty. may remove check_position.
add a check_game to find a winner, a game_over? boolean, and moves for X and O.
add 4x4 grid. rework command line. add 4x4 to web app.