Prompt the user for login/pass, if needed.

def prompt_auth(self):
        """Prompt the user for login/pass, if needed."""
        if self.username and self.password:
            return
        sys.stdout.write("Please insert your credentials for %s\n" % self.url.base_url)
        while not self.username:
            self.username = raw_input("Username [%s]: " % getpass.getuser())

        while not self.password:
            self.password = getpass.getpass("Password: ")