Fixes bug in moves method of SteppingPieces class that set color = opponent.
adds logic to play, disallowing playing opponents pieces.
changes filename from chess_game.rb to chess.rb
Adds additional logic for playing.
separates logic in play method of Game class.
adds save functionality.
refactors render, adds unicode functionality.
refactors all_moves and all_valids.
Adds pawn promotion.