Authenticates your user and returns an auth token.

        :param str username: Hummingbird username.
        :param str password: Hummingbird password.
        :returns: str -- The Auth Token
        :raises: ValueError -- If the Authentication is wrong

def authenticate(self, username, password):
        """Authenticates your user and returns an auth token.

        :param str username: Hummingbird username.
        :param str password: Hummingbird password.
        :returns: str -- The Auth Token
        :raises: ValueError -- If the Authentication is wrong
        """

        r = self._query_('/users/authenticate', 'POST',
                         params={'username': username,
                                 'password': password})
        if r.status_code == 201:
            return r.text.strip('"')
        else:
            raise ValueError('Authentication invalid.')