Added new feature: Maintenance mode. If checked, non-Admins will not be able to acess the WordPress backend and they will see a customizable message instead. Useful if you want to perform some maintenance work to your website and you don't want your Editors to add or modify content before you finish. Admins are not affected and they can always login as usual.

git-svn-id: https://plugins.svn.wordpress.org/slash-admin/trunk@1054082 b8457f37-d9ea-0310-8a92-e5e31aec5664