Implemented Board#in_check?
Implement in_check? and in_checkmate? in board_class
Draft game logic
Implement functioning game play
Implement iterate_through_grid
Prevents moving opponents piece and picking empty square.
Implements stalemate
Implement standard chess notation for player input
Implemented informative exceptions for invalid_move_from_player
Implement HumanPlayer class, moved input methods
Privatized methods (mostly in Board)