Updates to daisy202-to-epub3, notably for audio+NCC DTBs

 * if there is no HTML content in the DAISY 2.02, the Nav Doc converted
from the NCC is added to the spine and treated like a content document
for media overlay production
 * update content directory from 'OEPS' to 'EPUB'
 * the navigation document is put in the content directory
 * Nav Doc can point to itself (`span` leaves are not allowed in a Nav
Doc)