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.

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.