Wrap the custom user classes in a conditional block.

This can be used by new Oscar installs.  It's the same as a the core
Django one but without a username.

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

These are simpler as they are guaranteed to have an email field which
Oscar uses for authentication.

* 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

Example sites now sit in their own folder (sites) for easier
organisation.