editor: add fullscreen mode

Hi,

Please review my "fullscreen mode" feature (added to the view menu).

An example of this code is running here: http://wikischool.org/three.js/editor/

I am hoping to add some more features to this interesting project (as time permits), especially for "presentational work".

Thanks,

Walden
This is great! I think I would inline the code here directly and avoid having a `Fullscreen.js` file though.