see full commit comment (tests, bugs, validation)

- added schemas to xml catalog
- html rng:
  - renamed from html5-strict.rng to nordic-html5.rng
  - allow style in head
  - moved all schematron rules into nordic2015-1.sch
  - require header element in single-html books
  - allow navigation documents as content documents
  - cover must be top level section
  - allow div as pagebreak
  - allow h1-h6 (instead of just h1) as title (as in a poem)
  - made the table style attribute optional
- created nordic2015-1.sch based on tpb2011-2.sch
  - still very much a WIP
- renamed px:nordic-dtbook-validate.validate to px:nordic-dtbook-validate.step
- added report to epub3-to-dtbook script
- added epub3 / html / dtbook validation to epub3-to-dtbook
- epub3-to-html now creates a header element at the top of the body element, containing what will become the doctitle, covertitle, and docauthor elements
- epub3-to-html adds xml:lang to the top-level html element, based on the dc:language metadata
- epub3-to-html: output fileset only contains one instance of each file if a file occurs multiple times (for instance images used multiple times)
- created px:nordic-html-validate.step which validates a document according to both nordic-html5.rng and nordic2015-1.sch
- implemented a pretty-print XSLT for easier debugging
- fixed deep-level-grouping.xsl which nested the levels incorrectly
- epub3-to-dtbook.xsl
  - ability to call the "attrs" (and similar) templates without including any classes (set except-classes to '*')
  - added dtb:uid to metadata
  - made dublin core metadata lower case
  - don't convert the 'viewport' meta element or any element which does not contain both a name and a content attribute
  - change track:Guidelines to '2011-2'
  - moved pagebreaks directly following a headline to before the headline
  - z3998:author becomes `<author>` if not in header, otherwiser `<docauthor>`
  - notes and noterefs does not get a class attribute
  - all lists become preformatted and prepended with "• "
  - thead, tbody and tfoot is not used; replacing with their tr children
  - remove elements: link, colgroup, col
  - replace with p: address, annotation, epigraph, covertitle, bridgehead
  - replace with span: a, byline, dateline, dfn, cite, abbr, acronym, bdo, sent, w, annoref
  - replace with code: kbd, samp
  - added xsl:message for disallowed elements etc.
- navigation document gets a epub:prefix attribute so that it can be used as a content document
- automatically generated metadata: `<meta charset="UTF-8" />`
- automatically generated metadata: `<meta name="viewport" content="width=device-width" />`
- automatically generated metadata: `<meta name="track:Guidelines" content="2015-1" />`
- meta id not copied from opf to html
- made DTBook sample valid
- made EPUB3 sample valid
- added valid intermediary HTML
- added XSLT tests: deep-level-grouping
- added XProc tests: dtbook.validate, epub3-to-html.convert, html.validate
- updated XProc tests: epub3-to-dtbook, html-to-dtbook.convert