Needed when your site starts supporting multicurrency things

To support multi-currency sites.  This commit also made the order totals
a mandatory input for creating an order, which meant quite a few tests
had to change to support the new API.

Plus provide a currency property on the basket itself.

Should have checked this before pushing before...

Split into unit, integration etc.

Use charge_incl_tax instead of basket_charge_incl_tax().  This is
clearer.

Backwards compatible versions of the old methods are kept for now.

Related to #805

- Restructure into unit and integration tests.
- Add more tests for the various basket properties.

This splits the logic nicely into separate classes that can be mixed
together to create the strategy that you want.

Hmmm, this turned out to be a monster commit.  This change allows the
basket to be able to correctly calculate prices including tax.

It also requires a whole load of test changes since all baskets now
require a strategy instance to be assigned.

Rather than a StockRecord instance.  This makes it much simpler.

Changes to stock validation are stricter now.

This are to replace the partner wrappers.

Use field validators and model clean methods instead of forms.

Include new methods for voting up and down.

Add more tests.

Methods like clean_title don't get called dynamically.  Have moved
cleaning logic to the form which seems a more natural place.

Introduce some new test folders.

Plus reorganise tests

Validation is based on wikipedia list of postal codes [1]. Formats were
converted into regexes for countries.

1: http://en.wikipedia.org/wiki/List_of_postal_codes

Fixes #753

Fixes #745

Bad index in `last_line = covered_lines[-1][1]` (previously [-1][0])
caused rounding error. When you have 3 products for £7 and benefit 3
things per £20, each product would get discount £0.33 which doesn't sum
up to £1. The existing code handles this but bad indexing makes it fail.
A test case reproducing the issue was added.

Fixes #742

This change adds a uniqueness constraint on user addresses to prevent
duplicates.  A few changes follow:

- The user address forms in account and checkout are modified to take a
  user as a constructor arg so uniqueness can be checked.

- The assignment of a user when creating a new address has been moved
  into the form.

Also I moved the generate_hash fn up to the Address superclass as it's
possible that it might be used in other Address classes.

If parent is given, only parent descendant categories will be returned.

Fixes #741

- Add more links
- Add event details to line detail page

This allows updating code in both places, if necessary. Getting Django
1.5 support would be painful otherwise. This also removes the circular
dependency of django-oscar-testsupport on Oscar.

The requirements were merged. The imports were updated
accordingly. Unused imports in the touched files were removed. No
further changes.

* Tidy implementation of validator
* Tests improved and moved to tests/integration (as they call resolve
  internally)

As they save stuff...

Without a value, applying an AbsoluteBenefit raises a TypeError.

Fixes #670

The auth backend now gracefully handles situations where there are users
with the same email address.

Introduced in 7fd62f2af0.

This change applies the discount to all matching lines in the benefit
range, weighting the discount by the line prices.  The old way was to
iterate over each line and try and apply all the discount.

This new way means that applying an absolute benefit consumes more
products from the basket than the old way, which may possibly have some
confusing side-effects.

This change avoids an issue where customers can get a free product if
they return most of the ordered products apart from the one that was
discounted to zero.