Plus some general reorganisation.

Fixes #608

See http://djangosnippets.org/snippets/2533/

OSCAR_REQUIRED_ADDRESS_FIELDS allows setting which fields are required.
The use case is that post codes are expected to be required, but many
countries don't actually use them.

They really don't belong in /contributing.

Was posted on the mailing list.

Building the sandbox was explained in multiple locations, and in
slightly different ways. The sandbox now has a separate document.

my_model_customisations_arent_picked_up now lives in a subsection of the
recipe how_to_customise_models.

The old conventions.rst is now merged into the new coding-style.rst.

The old search.rst has been dropped as it seems outdated.

They were getting quite long. They've been split up in several
subsections. Some of them are borrowing heavily from the Django docs.

Also grouped promotion settings together.

Building on the work of @ArtS

/howto/index.rst was pointing at the wrong file. Renamed the file to
maintain naming consistency.

Yet another project is hard to maintain, and it's mainly used for
internal use. Removing it reduces the number of options of how to get a
Oscar project running, hence making it easier.

To set a base for upcoming docs changes, the docs directory structure
has been streamlined to follow a simplified version of Django's docs.

The old _draft folder has been removed. Both files from there and the
current docs that don't fit nicely into the new structure are in _old/
for the moment.

This allows partners to be identified without using a name that might
change or be translated.

Unfortunately, this involves a few tricky migrations to move data around
in the correct order.

Fixes #434 but reverts work done for #354

More focus on integration into root urls.py

Replaced legacy 'shop' name with 'application'

The docs contained reference on how to keep template tags working after
overriding an app, but this information is now obsolete as they've been
moved to oscar/templatetags. Thus, this info is removed.
It is replaced by similar instructions on how to keep the Django admin
around.

* Use `code-block` to see if that renders differently

See https://groups.google.com/forum/#!topic/django-oscar/-F1rEUX-RZA