Fix for missing xml declaration of content.opf and toc.ncx

Lack of xml declaration causes failed ePub validation with Epubcheck
Version 3.0.1
Fix for dc:creator and dc:date tag – make it as described in OPF 2.0.1 spec.

According to an example from OPF 2.0.1 spec:
<dc:creator opf:file-as="King, Martin Luther Jr." opf:role="aut">
        Rev. Dr. Martin Luther King Jr.
</dc:creator>

Regarding to dc:date change: Sigil throws  with xsi:type in dc:date.