Set this.Window and this.Document to null on unload. #2329

Fix for issue #2329

Tested in IE7, 8, 9 and 10, and the latest releases Firefox and Chrome.
Frees up large amounts of memory when navigating away from a page.
This was a trial and error fix for https://github.com/mootools/mootools-core/issues/2329#issuecomment-14787114.
Memory is freed up while navigating away from index.html anyway, but for a one page webapp this is an issue.
We are using this patch for over 2 years now. Seems to work ;-)