Fix Board#dup & finish Piece#valid_move
Create Board#move! Add Piece#valid_moves to Board#move
Create Board#render and Board#checkmate
Fix Board#checkmate
Create Game class.. Game now playable
Fix Pawn#moves
DRY Board#initialize
Create Board#build_board and its helper methods
DRY Board#move & redefine Board#[]
Finish DRYing Board
Replace SlidingPiece#horizontal and SlidingPiece#vertical with SlidingPiece#h_v_moves
DRY SlidingPiece#diagonal and SteppingPiece#moves
DRY Pawn#moves