The rest interface works without cookies and serves json

As discussed there with Lobo:
http://forum.civicrm.org/index.php/topic,5230.0.html

To get json, add &json=1 in the get chain.

Might be better to have :
/extern/xml.php
and
/extern/json.php

instead of
/extern/rest.php for both ?

The second file is a bare example of a class to use the rest interface. I intent to extend it to make is something good enough to be part of the civi distrib.