* Adds missing newlines
* Improves error outputs

Fixes #1428.

The command did not generate valid app configs. Now it does, and doesn't
raise PEP8 warnings anymore either.

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.

I think the Oscar upgrade/forking process for forked apps will get
simpler with the new migrations. One probably will be able to just fake
an initial migration, or something like that. But for now, copying
them across as we did before should be a safe bet.

Zero complaints by ./lint.sh now.

We ship some apps, e.g. search, without models.py. I think that
means it is not a valid Django app for Django < 1.7. But either way,
Django 1.7 definitely allows apps without models.py, so we need to
ensure we do the right thing.

Otherwise the ModelAdmins aren't discovered.

This makes it easier to customise Oscar. You can run this command to
quickly create local versions of an app that you want to customise. The
command creates the appropriate folder, __init__.py and models.py. The
only step requiring human intervention is to modify INSTALLED_APPS to
include the new app package.