Add 'superadminRole' config to Tiny

Adds config to allow a superadmin role access to all actions automatically, without the need to add Auth->allow() code in AppController and avoiding conflicts with some authentication mechanisms (see last post of http://www.dereuromark.de/2011/12/18/tinyauth-the-fastest-and-easiest-authorization-for-cake2/comment-page-1/).
Added test case method.
If is there anything I'm missing, just let me know.