Add config setters and setWhitelist

This PR fixes #491. It adds setters to ServerConfig, and uses those setters in GlowServer to set whitelist in Glowstone.yml.
Do we really need to save every change? Currently we don't have anything that does this, but here is a case where this would be undesirable:

```
for(int i=0; i<10000; i++){
    theConfig.set("something."+i, i); 
}
```