add support for username/password authentication

Per the rundeck docs here I implemented username/password authentication:
http://rundeck.org/2.1.2/api/index.html#password-authentication

It was pretty simple since the design is already using a python-requests Session object for all calls.  I tested it with our local install of rundeck and works as expected.  Not sure exactly how I would implement a unit test for this since it depends on a rundeck installation with user auth set up.