Initialize chess pieces
Adds SlidingPieces#moves.
Adds SteppingPieces#moves and Pawn#moves.
Sets up basic board, partial valid moves functionality.
Sets up board with pieces in starting positions.
Adds board#in_check, board#find_king and board#pieces.
Adds Piece#render, Board#render and Board#display
Adds Board#dup Board#extract_positions. Updates Board#initialize.
Adds Board#checkmate?, Piece#move_into_check?, Piece#valid_moves, created Yaml pos.file and updated Board#move