epub3-to-dtbook: file structure, doctype and xml declaration

- implemented px:fileset-move (changes fileset base URI without changing the file hrefs; also updates in-memory base URIs)
- epub3-to-dtbook: output DTBook is saved as `{dc:identifier}/{dc:identifier}.xml`, fixes #13
- epub3-validator: changed required content folder from "OEBPS" to "EPUB", fixes #17
- epub3-to-dtbook: images are now moved from the "images" folder and into the same folder as the output DTBook, and all image references are updated (`img/@src` and `object/@data`), fixes #14
- epub3-to-dtbook: doctype is now included, fixes #11
- epub3-to-dtbook: made sure that the xml declaration is included in the result, fixes #10