Fixes bug in Board#dup. Eliminates Piece#move method, moving all logic to Board#move method. Adds a buggy Piece#move_into_check? method
Fixes another bug in Board#dup, which fixes Piece#move_into_check?. Adds untested Board#pieces method
Adds Piece#valid_moves which returns moves of a piece that filters out the moves that would leave that piece's color in check
Board#move is now Board#move! New Board#move method calls Piece#valid_moves.
Adds Board#checkmate?