Fix #move_piece in Board class and add #moveable?. Add #perform_moves to Piece class.
Add InvalidPieceError.
Add #game_over? method.
Create HumanPlayer class with #get_piece_to_move, #get_move_seq, and #parse_input methods.
Fix #valid_move_seq? to duplicate the move_sequence passed to it so that move_sequence is not modified by the time the moves are actually performed by #perform_moves! in #perform_moves.