Allow using django CMS with custom admin site

Added a settings ``CMS_ADMIN_NAMESPACE`` (defaults to ``'admin'``) to use a non-standard ``AdminSite`` with django CMS.
Can we try to organize import statements in some logical order e.g. one recommended in PEP8 (http://legacy.python.org/dev/peps/pep-0008/#id17). It really makes reading and understanding source code much easier. I know that Python can handle randomly ordered import statements but "Beautiful is better than ugly." :-)