lots of updates to tests, xslts and schema

general
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* minor update to README
* added deep-level-grouping.xsl: will handle deeply nested headline levels (and similar)

html5-strict.rng
----------------
* the top-level doctitle, covertitle and docauthor elements now maps to a header element in the single-page html equivalent. cover and titlepage sectioning elements are no longer created or checked during conversion
* book/frontmatter/doctitle maps to body/header/h1, while all other doctitles maps to p
* all covertitles maps to p
* docauthors must now have a class of 'docauthor'
* covertitle no longer creates a meta element

dtbook-to-epub3.xsl
-------------------
* no longer create cover or titlepage based on doctitle/covertitle/docauthor
* "jacketcopy" is now interpreted as "cover"
* excluded a bunch of classes that were added during conversion
* if caption precedes first img, collect captions in figcaption at start of figure; otherwise collect captions in figcaptions at end of figure
* create empty divs with img-caption classes for img elements without a corresponding caption (only when in a imggroup and at least one other img has a caption); this allows us to map the caption directly to the img if converting back to dtbook.
* precedingemptyline class is removed (a preceding hr is already added)
* doctitle becomes a h1 when it's a direct child of frontmatter; otherwise it becomes a p
* docauthor class added to docauthor paragraphs (improves conversion back to dtbook)

epub3-to-dtbook.xsl
-------------------
* rename lang attributes to xml:lang
* excluded a bunch of classes that were added during conversion
* <dtbook:meta name="dc:title"/> is constructed based on <html:title/> unless <html:meta name="dc:title"/> exists, in which case html:title is discarded
* fix: template matching link didn't work
* including the rev attribute on a elements
* pagebreaks can now also be div, not just span
* including the page attribute on pagenum
* prettier id attributes
* if figcaption is at start of figure; make captions precede their corresponding img elements; otherwise let them follow
* docauthors must now have the 'docauthor' class
* h1-h6 elements are given names based on their level, not based on their name
* changed name of class from imggroup-caption to img-caption for img captions

dtbook-to-epub3.xspec
---------------------
* a bunch of updates to the inputs/outputs according to changes in the markup and schema

epub3-to-dtbook.xspec
---------------------
* a bunch of updates to the inputs/outputs according to changes in the markup and schema
* added a lot of tests for different kinds of elements; only lists and tables remaining