Add customisable dashboard navigation

I did some work on our Oscar demo site last week and made the dashboard navigation customisable as part of that. I moved a slight modification of this into a PR for further discussion.

The idea behind this is to move the hardcoded navigation into the settings file, or more precisely into ``oscar.defaults``. I have put a sample definition into ``oscar.defaults`` for now but that will break the navigation in existing installations which might not be a good idea. This can be prevented by leaving the ``OSCAR_DASHBOARD_NAVIGATION`` empty. To prevent previous projects from breaking we could make this the default in ``oscar.defaults``. 

This is just my suggestion. Any thoughts or suggestions? I am especially uncertain about the way I handle creating the navigation in the ``DashboardApplication`` constructor. Any better ideas?