legacy mode, image series, ++

# Script options

* dtbook-to-epub3: added "no-legacy" and "strict" options
  * no-legacy can be disabled to allow migration of existing dtbooks
  * strict can be disabled to allow non-nordic countries to try the migrator (should also be disabled when in legacy mode)
* dtbook-validate: added "no-legacy" option (same as for dtbook-to-epub3)
* epub3-to-dtbook: added "strict" option (same as for dtbook-to-epub3)
* epub3-validate: added "strict" option (same as for dtbook-to-epub3)

# Under the hood

* added nordic2015-1.strict.sch - a schematron which validates "extra strict" stuff such as requiring pagebreaks in all files and only allowing nordic dc:publishers. Can be disabled using the option "strict".
* require 'dc:identifier' in all content documents
* allow 'figure' as html flow content
* changed sidebar from aside/section to aside/figure (depending on `@render`)
* fixed image series markup in RNG
* don't check what language is used in nordic2015-1.opf.sch
* don't check what publisher is used in nordic2015-1.opf.sch
* added filename tests to OPF
* allow <span class="lic"/> as sibling to block content
* changed prodnote from aside to section
* require content in alt attribute for meaningful images, even for non-swedish/non-english content
* require empty alt attribute for irrelevant/stylistic images
* require 'image-series' class for figure image series
* require figure with class='image' wrapping around meaningful images
* using section instead of aside/prodnote for cover (jacketcopy). Still maps to prodnote in dtbook
* allow more epub:types
* generic dtbook abbr and acronym now maps to nordic dtbook span class=truncation/initialism/acronym which maps to html5/epub3 abbr epub:type=z3998:trunctation/z3998:initialism/acronym
* renamed class "pronounce-no" to "initialism"
* tpb2011-1.sch: checking language code syntax instead of actual language code values
* dtook-legacy-fix.xsl added which cleans up old dtbooks and makes them conformant to tpb2011-1.sch
* dtbooks older than version 2005-3 are upgraded when in legacy mode
* added validation of title tags, it should now be equal to the publication title; not the first headline in the document
* sidebar not using render-required/render-optional classes anymore; intent is implied by whether it's a aside or a figure
* added parameter to epub3-to-dtbook which can be set to true to preserve links in the result (instead of converting to span); not exposed as xproc/script option yet
* mapping z3998:halftitle-page to halftitlepage and z3998:title-page to titlepage
* only a predefined set of classes is allowed in the resulting DTBook; easily controlled at the top of the XSLT
* preserving metadata with nordic: prefix in html-to-opf-metadata.xsl
* fixed make-uris-relative-to-document.xsl (easy to implement now that we require all content documents to be in the same directory)
* including dc:identifier in single-page html when converting from epub3
* improved filename generation; they are now as in the requirements: [identifier]-[xxx]-[type].xhtml
* updated tests and test documents according to changes in converter
* fixed build version message
* allowed empty pagebreaks (TODO: check if this actually works)
pagenums, empty documents, cover, fulltitle, xhtml, doctype, chapters

* made sure that DTBook pagenums are converted to HTML divs and spans according to the content model (fixes #59)
* allowed for content documents with only a headline (fixes #61)
* added "cover" to allowed epub:types on body element (fixes #42)
* commented out nordic246 which restricted the use of fulltitle to the first header element (fixes #60)
* XHTML5 files are now loaded as XHTML, not as XML
* added support for doctype validation in epub3 validation, but disabled it due to p:store not supporting `<!DOCTYPE html>` yet (see #83)
* updated px:fileset-store to the most recent version in the Pipeline 2 repository (pipeline-modules-common), and made sure that all HTML content documents are stored as xhtml, not xml (fixes #81)
* automatically indented all content documents when they are stored
* dtbook-legacy-fix.xsl: allowed classes on level2 when wrapped in level1 with class="part"
* dtbook-to-epub3.xsl: default epub:type for `<body>` should be "chapter" for content documents in bodymatter