Fixed AdminController $this issues

I got many messages like these:

[Fri Sep 06 11:14:07 2013] [error] [client 127.0.0.1] PHP Fatal error:  Using $this when not in object context in /Users/llunesu/Sites/gitlist/src/GitList/Controller/AdminController.php on line 172

All fixed by changing the methods to static and using the full class name when invoking. (Even self:: wouldn't work!)