fork_app command: Handle apps without models.py

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.
Update docs for fork_app command

The recently introduced management command makes customising easier, and
the docs have been updated accordingly. The how-tos could still do with
updating to the new command, but at least the main docs are current.
Report Oscar version via tracking pixel

This was meant to be added in 0.7, but forgotten. Now, the Python,
Django and Oscar version in use are reported if OSCAR_TRACKING is
enabled. This helps Tangent to make informed decisions about what
versions of Python and Django to support, and how long to maintain older
Oscar versions.
Use inline formsets in admin for weight-based shipping methods

This couples the methods and their bands together, and allows editing of
all bands in one page.
Respect ordering of recommended products

The through-model ProductRecommendation has a ranking field, which is
never considered. Adding it to the Meta class ordering fixes that.