Specify the number of consecutive pieces to win
Split up _validate opening to use _is_valid_opening
Add another get_piece_at_opening test
Add _create_from_picture and __str__
Add _get_piece_at_opening_or_none
Add win-detection stub
Fix inverted rows bug in _create_from_picture and __str__
Add horizontal win tests and checks
Makes progress for #7
Remove an unremoved debug flag