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.