use "-" in navdoc for unnumbered pages and fix imggroup/figure content model

fixes #58
fixes #95

 - nordic-html5.rng
   - allow both div and span for pagebreaks in figures
   - require pagebreaks and prodnotes to be placed after the image in image figures
   - disallow trailing pagenums and prodnotes in image-series; they must be placed before all the image figures

 - nordic2015-1.sch
   - added suffix to difference nordic253 assertions for easier distinction (a,b,c,d)
   - assert that there is at least 2 image figures in a image-series
   - added nordic260a: The first element in a figure with class="image" must be a "img" element
   - added nordic260b: Content not allowed between or after image figure elements

 - dtbook-to-epub3.xsl
   - fixed imggroup/figure mapping (see #95)
   - only add title attribute to pagenum if the pagenum has a value (allows for unnumbered pages)
   - page type set to "special" if unnumbered

 - epub3-to-dtbook.xsl
   - fixed imggroup/figure mapping (see #95)

 - updated tests and test resources in order to test and be valid against #58 and #95
 - removed epub3-nav-utils (it's now included in the main dp2 distro)