upgrade Asciidoctor EPUB3 to fix multibyte char issue; upgrade Saxon

- Asciidoctor EPUB3 uses /\uFEFF/ to match BOM in gsub
- upgrade Saxon to workaround conflict with Nokogiri xpath
- set safe mode to safe in epub3 backend test (required atm)
- set imagesdir to images in epub3 sample document (required atm)