Fix Author to be automatically created on User creation

This is done by using post_save and changing the Author model to
use User as the primary key, explained here:

http://stackoverflow.com/a/18265192
Fix some of the unit tests

- SERVERONLY fix changed the posts that utestuser1 could see
- filter authors by GUID (which should be what is sent in the
  POST requests anyway), and not by ID
- filter posts by GUID