Login on coursera.org with the given credentials.

    This adds the following cookies to the session:
        sessionid, maestro_login, maestro_login_flag

def login(session, username, password, class_name=None):
    """
    Login on coursera.org with the given credentials.

    This adds the following cookies to the session:
        sessionid, maestro_login, maestro_login_flag
    """

    logging.debug('Initiating login.')
    try:
        session.cookies.clear('.coursera.org')
        logging.debug('Cleared .coursera.org cookies.')
    except KeyError:
        logging.debug('There were no .coursera.org cookies to be cleared.')

    # Hit class url
    if class_name is not None:
        class_url = CLASS_URL.format(class_name=class_name)
        r = requests.get(class_url, allow_redirects=False)
        try:
            r.raise_for_status()
        except requests.exceptions.HTTPError as e:
            logging.error(e)
            raise ClassNotFound(class_name)

    headers = prepare_auth_headers(session, include_cauth=False)

    data = {
        'email': username,
        'password': password,
        'webrequest': 'true'
    }

    # Auth API V3
    r = session.post(AUTH_URL_V3, data=data,
                     headers=headers, allow_redirects=False)
    try:
        r.raise_for_status()

        # Some how the order of cookies parameters are important
        # for coursera!!!
        v = session.cookies.pop('CAUTH')
        session.cookies.set('CAUTH', v)
    except requests.exceptions.HTTPError as e:
        raise AuthenticationFailed('Cannot login on coursera.org: %s' % e)

    logging.info('Logged in on coursera.org.')