Refactoring:

* Shared now exports fetchItem (instead of getItem) and openURL
* fetchItem has different parameters than getItem and includes
  some logic formerly in the ODT and Docx writers
* getItem still used in SelfContained
EPUB writer:  Download images at URLs.
Revised EPUB writer given changes in header attributes.

We need to ensure that all headers have attributes, since
this is no longer guaranteed by hierarchicalize.
Explicitly given headers remain the same.