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"
 
I'm personally used to the string version. What's wrong with this?

```
try{
    // Can't remember if this throws an exception or not. It's too late in the night. 
    return Difficulty.valueOf(config.getString(ServerConfig.Key.DIFFICULTY).toUpperCase());
}catch(Exception e){
    // Return some default
}
```