Adds Deltas for sublcasses of SlidingPiece, adds Piece#off_board? method and started SlidingPiece#moves
Finishes SlidingPiece#moves, works for every subclass
Adds SteppingPiece skeleton, Adds Knight & King classes
Finishes SteppingPiece#moves
Adds Pawn#moves without ability to take
Finishes Pawn#moves