Initial stab at breadcrumbs

There's currently a lot of copy-pasting of breadcrumbs, this can be revisited once we have a better understanding of how we're going to handle navigation.

I've also tried to make template context variables more logical and consistent throughout.
add missing test for utils.model_app_label
Rename app-index view to app_index
Merge branch 'breadcrumbs' of git://github.com/AndrewIngram/django-admin2 into AndrewIngram-breadcrumbs