Fixed URL generation for books published as websites.

Originally, easybook generated relative URLs for books published as
websites. The resulting template navigation code was pretty complex
and therefore, easybook recently switched to absolute URLs prefixed
by the book slug.

I heavily use easybook to generate books published as websites and
this worked great. The problem is that books can no longer be
published as static websites. In other words, unless you browse
your book through a web server, you couldn't read it.

This commit reverts this situation and goes back to relative URLs,
that work not only locally but also with web servers.