Sketched out Chess skeleton
Added  Board#[], #add_piece, #dup, #empty, and Piece#initialize methods
Added Board#valid_pos?
Added Piece#valid_moves
Added Piece#move_diffs
started writing Piece#perform_slide, #perform_jump and added Board#move_piece!, #remove_piece
changed Board#valid_pos? to Board#pos_within_bounds?
finished Board#remove_piece
added HumanPlayer class and Game#build_move_hash
added HumanPlayer#get_pos, #get_sequence
hopefully wrote Piece#perform_moves! correctly
finished Game#play method
finished Board#make_starting_grid
added Piece#render