Better to use filenames which are commonly used in the python word then
custom ones.

Replace 'assert' with 'assert*' method.

Convert tests.unit.forms.widget_tests to use TestCase class.

Convert bankcard tests to use TestCase.

Convert tests.unit.logging_tests to use TestCase.

* 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.