Registering receivers is a tricky business pre-Django 1.7 as they get
registered at the same time when models get registered, but require the
sending model to already be in the app cache. It's surprising we don't
see more issues!

Recommendations:
* Avoid calls to get_model in modules that get called during app init at
  all cost
* Don't use get_model in your own code. Just import directly.
* Switch to Django 1.7 :)

This commit probably fixes the issue in #1205. But get_model calls to
fetch the category, product image and stock record will all fail,
because receivers listen to their signals.

This commit also moves the catalogue receiver into it's own receivers
module, because that's how we do it elsewhere. That won't make a
difference to the issues though.

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.

It's such a core concept, it deserves a dedicated document. Moved some
of the "why" from the how-to into it as well.