Adds online mode API

Currently, the server's online mode has been a protected, read-only setting that cannot be changed from in-game mechanics and programming, besides using hack-around Reflection. This PR is here to change that. 

There are various times in game when you might want to turn on / off online mode without restarting the server. One example of this would be the LilyPad plugin - which currently uses Reflection to handle this. With this new added API method, the added abilities of the API are extended which allows plugins to be even more creative than they were before - without breaking the rules with Reflection.

Besides - if a programmer wanted to do this before, they could have. This pull request just makes it easier and more official to change the online mode of a server without restarting.