Fixed z3998:author validation on titlepage, better validation

 * HTML RNG: commented out epub:types that have specific markup requirements from the list of types allowed everywhere
 * OPF Schematron (strict): emphasize that the unique-identifier, dc: namespace, and prefix attribute assertions is targeting the package element
 * HTML Schematron: disallow content inside pagebreaks (a sequence of spaces were allowed before this)
 * HTML Schematron: fix z3998:author validation to allow for it to be in multi-HTML titlepage as well as single-HTML
 * HTML Schematron: improve error text for nordic251
 * HTML Schematron: Added nordic262: Only fulltitle and docauthor are allowed on the titlepage
 * fixed test documents so that footnotes match the required content model (i.e. must be top-level)
Merge pull request #175 from nlbdev/fix-author-validation-on-titlepage-and-improve-validation-in-general

Fixed z3998:author validation on titlepage, better validation