get_model: Raise LookupError instead of ImportError

oscar.core.loading.get_model is a thin wrapper around Django 1.6's
get_model function. The latter returns None if the model can't be found,
whereas the Oscar variant loudly failed with an ImportError.
With Django's app refactor, most of that behaviour in Django has been
changed, and Oscar's get_model falls back to Django's
get_registered_model. But the latter raises a LookupError.

To avoid having to catch both errors, Oscar's get_model now raises a
LookupError no matter what Django version is used. This should have
minimal impact on backwards compatibility, as get_model isn't really
used outside of Oscar core.
Handle app configs in oscar_fork_app command

A custom app needs to inherit from Oscar's app config. This ensures the
correct app label, and that any startup code (receivers, etc.) is
executed.
Per the camp ground rule, the fork_app command has been slightly
refactored.