don't require type on footnotes/rearnotes ol, allow rearnotes in backmatter

 * allow rearnotes in backmatter
 * add error messages which lists allowed types for cover, frontmatter, bodymatter and backmatter
 * removed requirement that the ol element wrapping footnotes and rearnotes must have the epub:type "footnotes" or "rearnotes"; fixes #214
don't use z3998:production in cover

fixes #220
Merge pull request #227 from nlbdev/dev-20141123

drop z3998:production from cover, rearnotes in backmatter, foot-/rearnotes not required on ol