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

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.