Authenticate a session.

Arguments:

session      A clj-libssh2.session.Session object referring to an SSH
             session which has not yet been authenticated.
credentials  Either an instance of Credentials or a map which can be
             transformed into a Credentials object.

Return:

True on success. An exception will be thrown if ther session could not be
authenticated.

A datatype to represent a way of authentication and the necessary data to
use that authentication method.