Added fullscreen support for OS X.

Tried to also add http://alvinalexander.com/blog/post/jfc-swing/making-java-swing-app-look-like-mac-app to the MinecraftFrame. But since I truly am no Java developer, Google can only help so much ;)

And, I suspect that my small snippet is at the wrong place :P Just used the stuff from https://github.com/TechnicPack/Spoutcraft-Launcher/pull/46.