Allow more types, fix bug with deep section nesting, fix bug with note lists and validate navigation document heading references

 * added validation that makes sure that all headlines in the book are referenced from the navigation document
 * fixed conversion of note lists (again!), fixes #187 
 * fixed problem with using nesting depths of 6 or more in the sectioning hierarchy, fixes #254 
 * allow more epub:types in top-level sections, fixes #230. titlepage is still only allowed in frontmatter and part is still only allowed in bodymatter. Other than that, these types can be used in either frontmatter, bodymatter or backmatter: chapter, abstract, foreword, preface, prologue, introduction, preamble, conclusion, epilogue, afterword, epigraph, toc, toc-brief, landmarks, loa, loi, lot, lov, appendix, colophon, credits, keywords, index, glossary, bibliography, titlepage, halftitlepage, copyright-page, seriespage, acknowledgments, imprint, imprimatur, contributors, other-credits, errata, dedication, revision-history, case-study, answers, assessments, qna, practices, footnotes, rearnotes