Add a GET request for searching local authors

This is intended to be used by remote servers to search our authors.

URL: <ourhost>/api/search?query=<querystring>

Empty or missing query string returns all authors. Authors are returned
as a json list of the following format:

[
 {"url": <author url>,
  "host": <our host>,
  "displayname": <author username>,
  "id": <author GUID>},
  ...
]

API tests have been updated.
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
Clean up author/models.py

Removed some old code, renamed post_save method
fixed merge conflct
put changed to accept author id as query string, and put api tests work finally
api tests now pass again, and lots of bug fixes with post rest api