ZCML directive for overriding realm configuration

See the readme:

The realms are by default configured in the database through the control panel.
When copying the database from a production environment to a staging environment
the realm configuration is copied too, which could result in publishing from
the staging editorial site to the production public site, which is very bad.

For solving this issue it is possible to override the realm configuration with
ZCML, so that it can be configured also using the ``zcml-additional`` option of
the buildout.

```xml
<configure xmlns:publisher="http://namespaces.zope.org/ftw.publisher">

    <include package="ftw.publisher.sender" file="meta.zcml" />

    <publisher:override-realm
        url="http://localhost:9090/site"
        username="publisher-user"
        password="publisher-password" />

</configure>
```

/cc @maethu