A bunch of schema improvements, some bugfixes, ++

 * major cleanup of Relax NG schema
   * class="noteref" is now allowed. fixes #145
   * deleted old DTBook comments (they were there to help compare during development of the schema)
   * renamed several definitions to more HTML-sounding names
   * restructured and simplified several places
   * added requirements for epub:type and class values directly into the Relax NG schema; previously they were only in the Schematron, but having them in the RNG should allow for both better validation as well as decent content completion if you associate the RNG with the document in your XML editor
   * moved rearnotes to bodymatter
   * fixed allowed class values for cover and flaps
   * allow nested lists
   * allow pagebreak in list item block context
   * added render-required and render-optional to list of classes (for easier content completion in editors)
   * replaced all references to "z3998:covertitle" with "covertitle"
   * HTML: removed much of tpb123 which is mostly relevant for DTBook
   * HTML: disabled tpb134
   * disabled validation of rearnote and footnote placement while waiting for #160
   * require class="title" on fulltitle
   * require class="docauthor" on main author
 * commented out `[tpb135] Element <sch:name/> is only allowed in poem context` as it doesn't really make sense in HTML
   * fixes #133
   * replaced with rule; `[tpb135] z3998:author is only allowed inside a poem`
 * fixed: create titlepage based on single-HTML `<header>`
 * replaced references to epub:type="author" with epub:type="z3998:author" (there's no "author" in the default vocab)
 * properly mapped footnotes and rearnotes which fixes #161 (at least in terms of DTBook/HTML; HTML splitting still remains, see #160)
 * note is mapped to either aside or list item depending on what kind of note it is (rearnote/footnote becomes list item)
 * adding notebody class to all notes in HTML
 * put list marker into a `<lic>` when needed
 * updated tests as necessary