FullScreen Addon

Hi Marjin,

this is an enhancement of this pull request of angelozzer: https://github.com/marijnh/CodeMirror/pull/1490

You said that there would be problems when the editor is inside a relative or absolutely positioned element. Since I had this problem and I solved it by moving the CodeMirror around in the DOM tree I would be happy if it would be added to the next release. 

I appreciate your comments and hope you like it!
Philipp