Better to use filenames which are commonly used in the python word then
custom ones.

The former throws a deprecation warning and causes the test suite to
fail.

Most email servers don't respect capitalisation, and many users don't
know about it. So Oscar now does what the rest of the world does, and
ignores the capitalisation when looking up an email address.

Fixes #1179.

We need to bump the version of django-dynamic-fixture to get the test
suite to run under Python 3. They've dropped Python 2.6 support in the
same release.

Previously, the test suite ran with a custom user model for Django >=
1.5, but it didn't subclass Oscar's AbstractUser.  This commit adjusts
the custom user model to subclass Oscar's so we can test functionality
within the custom user class.

This does require a change to the way tests authenticate as we don't
actually use the username field.

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