Adds Board#[] and Board#[]= to set pieces
Fixes @board[pos] to accomodate Board#[] and Board#[]=
Adds Board#length to return length of @board ivar in Board
Corrects @board positions to accomodate Board#[] and Board#[]= methods; Returns 'false' for invalid jumps per instructions; Removes debugging 'p' in Piece#get_middle_piece
Corrects Piece#perform_jump to put jumpee into board to get color
Creates Piece#set_pieces to set pieces during a valid move