Added GetVersion in Game!

Needed for the changes I made in the mod!
https://github.com/thomas15v/Sponge/commit/33c75c690f91aa41077aacdcb14b3465e17684dc
Just as a reminder / todo for the future: Maybe add a more detailed javadoc to that method; What does it return exactly?
A [semver](http://semver.org/) valid version string? Something like "vX.X.X-SNAPSHOT (FML X.X.X)"?

Would be helpful for plugins that need to know how to parse the game version when checking if it is compatible with the server, (which might also be nice to have as a general feature, /cc @sk89q @DDoS).