v0.1.17

OPDS: fix download auth loop, clickable authors/genres, richer entries

- Send a WWW-Authenticate: Basic challenge from /Images/* when there is no
  session cookie, so OPDS clients (MoonReader) stop looping on the password
  prompt at download and can load covers.
- Make the OpenSearch template an absolute URL — mobile clients often fail
  to resolve a relative one.
- Include format and size in OPDS entry descriptions.
- Book page: authors link to the author's books, genres link to the genre
  shelf (book details now expose author ids and genre codes).