Get board rendering. Implement [] and []=.
Add reference to board in Piece
Implement Board#empty_square? and Piece#perform_slide and Piece#move_diffs
Implement Piece#perform_jump
Implement Piece#pos=
Implement to_s, inspect. Debug valid_jumps, perform_jump, etc.
colorize squares
correct the color placement
Start to add in fillboard option
add optional fillboard option for Board
implement Board#dup
implement Piece#perform_moves!
implement Piece#valid_move_seq?
Implement perform_moves
Implement Game class
implement HumanPlayer
Debug Game a bit
refactor some and delete scrap
Refactor is_valid_move?