This allows importing a class or attribute by the reasonably common
string notation, which is useful for e.g. settings. When using settings,
we should prefer absolute imports over using get_class, because that
limits us to modules in INSTALLED_APPS.

* It can now be specified in OSCAR_DASHBOARD_DEFAULT_ACCESS_FUNCTION
* nav.py and menu.py were split up so that Node can be dynamically
  loaded and hence overridden

I recently needed to alter the access function and realised it's not as
easy as I thought.