added test for prodnote mapping issue in cover

fixes #151 as of PR #205 (this commit adds tests)
simplified frontmatter/bodymatter/backmatter types in schema

 * added preface to frontmatter, fixes #189
 * added introduction to bodymatter, fixes #196
allow linegroups to be divs when they do not contain headlines
added error message for missing epub:type on footnotes/rearnotes ol

fixes #187
Report error if the navigation document occurs in the spine

fixes #201
require id on pagebreaks
Merge pull request #208 from nlbdev/issue-151-187-189-196

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