Enhance order dashboard

- Add more links
- Add event details to line detail page
Rework shipping event validation
Introduce payment event subtotal calculation
Rework the shipping event testing methods
Document how order model can be extended to validate shipping events
Add verification of payment events
Add migration for order address models

Picks up the postcode being NULLable in the address superclass.
Rename date field of event models to date_created

To be consistent with the rest of Oscar.
Further document EventHandler class
Add reference field to payment event

This is useful for linking between payment events and transactions.
Add FK to shipping event from payment event

This allows them to be linked together which is often how the occur in
the real world.
Merge pull request #731

Conflicts:
	docs/source/releases/v0.6.rst
	oscar/apps/order/processing.py
	oscar/templates/oscar/dashboard/orders/line_detail.html
	oscar/templates/oscar/dashboard/orders/order_detail.html
Fix bug introduced in ccd1635 re product factory

The backported version didn't explicitly create the ProductAttribute
instances before assigning and so they didn't save.
Rewrite address migrations from 352844ca54

- Also rename field to display_order as that makes more sense.
Fix template logic error
Rework order detail template

- Add new block for payment tab
- Rename transactions block
- Provide transactions in get_context_data
- Better handling of empty iterables
Add local order app to demo site

- Fix payment event reference issue
- Upgrade django-oscar-datacash
- Handle changes to bankcard obj
Replace line_actions block in demo order dashboard
Add shipping event type fixture to demo site
Add simple order processing to demo site

- Only handles Datacash payments at the moment