Create a factory module per app and move them under the new
oscar.test.factories module.

Adds new functional tests for ShippingAddressView, PaymentDetailsView
and ThankYouView.
It's the second part of #1645.

Notes

* Adds a custom template to be able to test some functionalities of
  PaymentDetailsView
* Adds an helper to the CheckoutMixin

* Uses factory-boy factories
* Adds helpers to the CheckoutMixin helper
* Adds plenty of functional tests
* Adds HTML IDs to some forms to be able to easily reference them
  from our tests

@maikhoepfel:
This commit is the squashed version of #1645, but it
does not contain the template changes. The unused
ShippingAddressView code was already cherry-picked across in 549decb.

I also fixed some PEP8 issues.

We have two methods doing something very similiar, so I'm removing the
one with the worse name.

Thanks go to @itbabu for spotting it in
https://github.com/django-oscar/django-oscar/pull/1645#issuecomment-71896876

The previous skip condition for shipping bypassed the shipping method
view but this is required as it sets the shipping method code.

When ordering a basket of free products, or with vouchers giving 100%
discount, we should not collect payment details.

To achieve this, I've added a check to the CheckoutSessionMixin which
calculates the order total and decides whether payment is necessary.

The modified test failed because the OrderCalculator called by
check_payment_is_necessary accessed other properties on the shipping
method. We now just mock a valid shipping method, which still achieves
the aim of ensuring there's only one.

A bit OCD, but I noticed that the checkout forms were an exception in
using a hyphen so I've cleaned up all IDs to use underscores.

We no longer use Django's test client.

- Comment the session manager better.
- Clean up the functional checkout tests.

It is now replaced with a checkout pre-condition that applies to every
checkout view. If someone's basket becomes invalid while they are in
checkout, then they will be redirected back to the basket page.