Use difficulty set in glowstone.yml

Allows user to change the server difficulty 0 (PEACEFUL),1(EASY),2(NORMAL),3(HARD) in glowstone.yml and take effect.
I personally wanted this implemented because 'Hunger now refills on Peaceful difficulty"
 
    public Difficulty getDifficulty() {
        return Difficulty.getByValue(config.getInt(ServerConfig.Key.DIFFICULTY));
    }

The reason is a string "EASY" isn't handled as well as the above code. Also this is what everyone is use to.