Oscar is becoming a more community-owned project. This commit reflects a
few organisational changes taking place.

This reflects the change in the repo URL.

This is the result of wild grepping through the codebase. Nothing too
exciting to report, really. create_product now sets a products structure
to 'child' if a parent is specified.

This includes two examples for product attributes.

Also fixes two "document not referenced" warnings.

Fixes #794.

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

- Expand the ref/apps docs. (Still lots to do here.)
- Split up the index page links for clarity.

Those documents lived in ref/, but really aren't reference documents.
Topics/ is a more appropriate place for them.

 - make sure all pages are linked from somewhere
 - prohibit double indexing of source code modules
 - remove old unused offers (we use GIT, no need to keep old files)
 - use local version of Oscar logo

Fixes #921

With details of new prices/availability structure.

We currently mention it in internals/sandbox.rst. I've added a note to
the dashboard app (ref/apps/dashboard.rst), the sandbox/urls.py and
expanded on the feature explanation on index.rst.

Fixes #816

Commonly used terms should be explained there.

Attempt at making it clearer what needs to be done for what kind of
customisation. Created a new topic that lines out the prepatory
steps that need to be taken, and moved duplicate information from the
how-tos into the new topic.

The old howto included out-dated info on how to translate Oscar. That
has been replaced by a link to the Transifex project.
It was the only howto left under the Contributing headline, which really
shouldn't be there (as there's an entirely different section in the docs
for contributing). As it is more of a topic anyway, and will hopefully
soon include documentation on translation style etc., it was turned into
a topic.

Fixes #608

They really don't belong in /contributing.

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

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

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.

* Converted to abstract models
* Added tests for conditions

Pulled template variables into a class context variable
Updated shipping method loading