Allow documents with no headlines, remove strict validation, improved messages for opf12

 * opf12 - allowed optional numbering after epub:type to allow producers to annotate with chapter numbers etc. if they so desire (for instance `DTB123-15-chapter-4.xhtml`, `DTB123-16-chapter-5.xhtml`, etc). This is not required, but optional!
 * opf12a - improved message with examples and explaination
 * opf12b - improved message with reference to epub3 and zedai vocabularies as well as suggestions for typos
 * opf12b - identifier part validated to make sure it corresponds with dc:identifier in metadata
 * removed the three "strict" requirements:
   * tpb43: Meta dc:publisher must exist and have value 'TPB', 'MTM', 'SPSM', 'Nota', 'NLB', 'Celia' or 'SBS'.
   * tpb124: All documents in the spine (except cover and titlepage) must contain page numbers
   * tpb131: xml:lang must be one of: 'sv', 'en', 'da', 'it', 'la', 'el', 'de', 'fr', 'es', 'fi', 'no' or 'is'
 * added requirement:
   * tpb124: The publication must contain pagebreaks
     * (at least one pagebreak in the entire publication, not one per document as before)
 * removed "strict" option from scripts
 * include documents that are not referenced from the navigation document when converting to a single HTML (fixes #203), this allows for (for instance) rearnotes with no headline or footnotes with no headline
 * added to translations.xml: de, de-CH, de-DE, fr, it, es