Some of them are rather basic, but it's a start.

Fixes #697

* Better wordings for the individual steps
* Mentioned necessary copying of migrations directory
* Fixed wrong prerequisites in examples
* Glossary typo fixed

Commonly used terms should be explained there.

- Added stubs for both apps
- Switched to using :automodule: for abstract models instead of
  individual :autoclass:
- Added instructions on creating a Partner and ProductClass instance
  when setting up a new Oscar instance

Plus some general reorganisation.

Fixes #608

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 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.

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.