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.

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.

Completed the models for the order package too.  Added admin files