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.
I'm not asking to do full PEP8 cleanup in this PR but to at last not make it worse. It costs nothing to put imports in the "right" order but could make django CMS more PEP8 compliant.