Abstract base classes for shipping models

This is a common and useful Oscar pattern, and there's no reason to not
use it for the shipping app.

They are not imported anywhere in Oscar and just provided for
convenience, so no imports had to be altered.
Drop legacy ShippingMethod name

Changing a unofficially deprecated model name is clearer than having
three places to get at the same model.

The name 'AbstractBase' is consistent both with our naming of other
abstract models, and lots of apps having a Base class.
Merge pull request #1296 from tangentlabs/fix/abstractshippingmodels

Abstract base classes for shipping models