get_classes: Allow importing from any module depth

Previously, get_classes could only import from modules one level below
the INSTALLED_APPS entry. This wasn't a problem so far, but is needed to
when Oscar's Application instances switch to using get_class to load the
views.
Switch all Oscar apps to use get_class for loading views

This makes overriding views possible without the need to first override
the app.
Add test for overriding views without overriding app
Update docs for dynamically loaded views

It's getting easier and easier to customise parts of Oscar; the
customisation section can do with a rewrite because of it. Added to my
to-do list.