add support for CSS widows and orphans

ported from the following commits in Webkit:

http://trac.webkit.org/changeset/141009
http://trac.webkit.org/changeset/137200
http://trac.webkit.org/changeset/123904
http://trac.webkit.org/changeset/123684

I've already made the changes in wkhtmltopdf/qt@0ec3313 and wkhtmltopdf/qt@d8c337a