All code handling email addresses has been update to use a new function
normalise_email, to have one consistent way of handling email addresses.

This function lowercases the domain part of an email address if it can
find an @, so it works on partial addresses as well.

The new model must have an email field but Oscar will work without
anything else.

* Emailbackend: Updated to use REQUIRED_FIELDS
* Demo and sandbox site now use compat.AUTH_USER_MODEL as well
* Added example custom user that gets tested for Django >= 1.5

Django 1.5 makes it optional, but given our Emailbackend being default,
that seems like the sensible way to deal with it.

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

Now it checks to see if an email is being submitted.

Before it was lower-casing everything first which prevented users being
able to login if their email address contained capitals.  We now only
lower case the host part as this is case-insensitive.

Fixes #505

We catch the resulting MultipleObjectsReturned exception and delete the
dupes.

Closes #295

tests

Uses a class-based reimplementation of Django's existing function-based login view. The view also handles registration at the same URL endpoint. It depends on the submit button having a name of "login" or "registration" to determine which form was POSTed.

Auth is done using email addresses rather than username (which is a randomly generated string). The registration process enforces uniqueness, but this won't be enforced anywhere else, so we need to be careful not to duplicate email addresses in the admin or something.