Remove support for Basic Auth

In the past, we used this to provide API authentication.

Now though, we are using Bearer token authentication which is far more flexible
and powerful: you can have per-app API users who can be granted permissions on
a needs-basis rather than every API client having blanket access.

This will constitute a breaking change and hence a major version.
Improve docs about User data model
Add a changelog
Merge pull request #23 from alphagov/remove_basic_auth_api_access

Remove support for Basic Auth