Pipeline 2 v1.8

A bunch of changes and improvements, mainly to get the scripts working with Pipeline 2 v1.8.

 * OpenDyslexic's LICENSE.txt is now included in the epub output
 * fixed issue with all opf items getting `linear="no"`
 * fixed problem with epubcheck report generation
 * `*-cover.xhtml` marked with `linear="false"` in OPF. fixes #75
 * validate that content files uses the doctype declaration `<!DOCTYPE html>`. fixes #83
 * added .travis.yml with language: java
 * allow epub:type="toc" in nav doc
 * use px:epub3-store
 * removed step/fileset-store.xpl and step/fileset-move.xpl (they are bundled with v1.8)
 * adding original-base to dtbook fileset when loading dtbook
 * removed fileset.in port when invoking px:html-to-fileset - (this should be added back in a later release of Pipeline!)
   * as a temporary workaround in html-to-epub3.convert.xpl, the original-href is fixed manually
 * swapped order of viewport and dc:identifier in html head
 * changed type of pagenum with id="page_4" from "normal" to "front"
 * added original-href attribute to test fileset in html-to-epub3.convert.xprocspec
 * added classes "num-with-space" and "num-word" to test documents
 * updated .gitignore according to restructured folder structure
 * fixed split-html.annotate.xsl (this bug: https://saxonica.plan.io/issues/1819)
 * fixed dtbook-to-epub3.xspec (viewport/dc:identifier switched place)
 * both 'rearnote' and 'rearnotes' must be used in rearnotes
 * only allow epub:type="toc" in frontmatter and backmatter
 * switched ordering of viewport and identifier in html head
 * removed aggregator POM; there's only one project anyway