Transform code to OOP style

Hello, I've attempted to fix issue #10 here, to transform the code into OOP style. The tests pass, so I hope it's okay.
How about moving this as well into the class, as `run()` so that the class would be invoked as shown below

    game = Hangman(words)
    while True:
        game.run()
        if playAgain():
            game = Hangman(words)
        else:
            break