translated default headlines for sectioning elements, tests for cover

 - added translations for untitled sections (en, no, sv, fi, da)
 - fixed assertion in nordic2015-1.opf.sch requiring cover to be secondary in spine
 - allow section without headline only in cover
 - generate headlines for use in the navigation document when headlines are missing
 - dtbook-legacy-fix.xsl: don't allow jacketcopy, precedingemptyline, precedingseparator, byline, dateline
 - epub3-to-dtbook.xsl: allow classes: frontcover, rearcover, leftflap, rightflap
 - added cover to test documents (DTBook, single-document XHTML, EPUB3) and updated tests accordingly

fixes #92