Create new SImple User Setup admin

It works by creating a wrapper around django-cms permissions.

The wrapper is centered around the ```Role``` model which inherits from the cms'  ```AbstractPagePermission``` (therefore having cms specific permissions) and which references django ```Group``` objects which contain the rest of the permissions.

Site specific global page permissions and site specific django groups are automatically created when adding a new site or creating a new role: 

* the auto-generated global page permission is cloned from the ```Role``` objects.
* the auto-generated group is cloned from the original ```Group``` referenced by the ```Role``` object

When giving an user a role within a specific site, that user is actually assigned to an auto-generated site specific group which is referenced by an auto-generated global page permission object that grants required rights to that particular site.