Allow non-linear spine items in epub3-pub-utils

- `px:epub3-pub-create-package-doc` adds XHTML Content Docs from the
  publication resources to the spine, as non-linear items. This follows
  a change introduced in EPUB 3.0.1
- any entry from the "spine" manifests can be marked with the `linear`
  attribute, which will be copied to the spine `itemref` elements.
Merge pull request #54 from daisy/feature/non-linear-spine-items

Allow non-linear spine items in epub3-pub-utils