Needed when your site starts supporting multicurrency things

This prevents issue where it gets passed as an order kwarg but then gets
created separately as well.

The new price_currency field was not explicitly being excluded.

- build_submission now has kwargs passed to it so it can extract
  information from any forms that might be in use.
- the place_order function has been modified to use the passed in user,
  rather than using self.request.user

To avoid duplicate log messages

The PaymentDetailsView.submit method now requires more things to be
passed in explicitly to create the order.

This allows payment processing to know the currency of the total.

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.

To solve dupliate numbering issue after merge from master.

Plus provide a currency property on the basket itself.

This means it's easier to apply taxes to the shipping method.  Without
this, the shipping method was reloaded each time it was needed which
made it difficult to apply taxes only once.

Taking lead from Meridian project.

This gets tricky when some lines have tax but some don't.  We don't look
for a warning when the tax is not defined.

I spotted that the order confirmation page didn't have a title saying
that it was a confirmation.  This fix addresses that and adds titles to
other templates to be consistent.

It now returns a OrderTotal instance which has price fields with- and
without tax.

We now check that a valid shiping method can be looked by its code, not
just that a code is set in the session.  This avoids the case of
silently reverting back to free shipping.

Fixes #801

Some method defaults have been removed to make it more explicit when
things are loaded.

I prematurely refactored without thinking through the implications.

Should have checked this before pushing before...

It was using the wrong template variables...

The core API now has an is_tax_known property.

Related to #805

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

Merging happens from middleware, before a strategy has been assigned.
If we use all_lines then this causes an error, so we deliberately use
the normal related manger method.

Conflicts:
	oscar/apps/catalogue/abstract_models.py
	oscar/templates/oscar/basket/partials/basket_content.html
	oscar/templates/oscar/basket/partials/basket_totals.html
	tests/unit/partner/model_tests.py

To make it easier to override in subclasses.
 Please enter the commit message for your changes. Lines starting

Fixes #766

To make it easier to customise small parts of the template.

Also adds a docstring to the is_discountable AbstractProduct

We're not sure if a valid id might ever be 0. And using all() is a
little less verbose.

That custom get_object method almost worked. Now it does.

Sometimes you need to use the session to determine the availability
shipping methods.

spelling corrected in comments