Add management command for forking an app

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.
fork_app command: Also create admin.py

Otherwise the ModelAdmins aren't discovered.
Merge branch 'forking-command'