Keep OEB NS in DAISY 3 OPF for spec compliance

Quote from the specs:

> "For upwards compatibility, the element dc-metadata in an OEB package
> is required to have an attribute of
> xmlns:dc="http://purl.org/dc/elements/1.1/" and
> xmlns:oebpackage="http://openebook.org/namespaces/oeb-package/1.0/".

See http://www.niso.org/workrooms/daisy/Z39-86-2005.html#DCM