Get EPUB to work with the editor

Changes:

- There was no navigation document so I added one at `/EPUB/xhtml/epub30-nav.html` and uncommented the line in the OPF (and changed the file extension)
- To be able to view the images in the editor I moved the photos directory into `/EPUB/xhtml/` for 2 reasons:
  1. That was the path specified in the OPF file
  2. That was the path specified in each HTML file

Some other notes:

- There must be a file at `/META-INF/container.xml`
- The editor will put new content into `/content/` and the file name will be a UUID
- The editor will put new images into `/resources/` and the file name will be a SHA1 hash of the contents and if it is a JPEG or PNG the file will also get an extension (so github.com can preview it)
- The editor also works on private repositories (I think)