Several bugfixes and improvements to preface, introduction, footnotes, linegroups, pagebreak ids, validation

 * require id on pagebreaks
 * Report error if the navigation document occurs in the spine; fixes #201
 * added error message for missing epub:type on footnotes/rearnotes ol; fixes #187
 * allow linegroups to be divs when they do not contain headlines
 * simplified frontmatter/bodymatter/backmatter types in schema
 * added preface to frontmatter; fixes #189
 * added introduction to bodymatter; fixes #196
 * added test for prodnote mapping issue in cover; fixes #151 as of PR #205 (this commit adds tests)