Migrate epub3-nav-utils tests to XSpec.

Also slightly improved the `nav-to-ncx.xsl`, adding typing to the
`f:make-content-src` function and making it receive `a` elements
as arguments instead of their `li` parents.
Support empty page breaks in html5-to-pagelist
Add a warning when page-breaks have no value
Merge pull request #31 from daisy-consortium/epub3-nav-utils

XSpec tests + small improvements to epub3-nav-utils