v3.0.0

• Fixed bug where castles and en passants were not registering
  • The special moves were being saved according to the position of the piece, but were being accessed according to the id of the piece.
• Moved castling ability logic to GameState rather than per-piece (#1)
  • The ability of the FEN string to exactly regenerate a board state implies that the castling ability for a color should be saved at the game level, not the piece level.
  • This change also replaced the has_moved property of Piece with the is_at_start_pos() method.