Add a test client.

This deals with issue #147 - upstreaming the client.

Some changes had to be made for it to work with in this format:

 - We don't know what website object to use, so that has to be passed in to the constructor.
 - Because User objects are "owned" by the applications using the framework, Aspen doesn't seem to "know" anything about Users - so I had to create an "add_cookie_info" function that will be overridden in gittip (PR on the way for that too...)