'Tranfer a cosigner\'s credits to another cosigner.
:param id: the id of the sending cosigner
:param recipient: the id of the recipient cosigner
:param otp: the one time password (of the sender)
:param signature_callback: a callback that signs a text message using xpubkey1/0/0 returning a compact sig'

def transfer_credit(self, id, recipient, otp, signature_callback):

    payload = {'otp': otp, 'recipient': recipient, 'timestamp': int(time.time())}
    relative_url = ('cosigner/%s/transfer' % quote(id))
    full_url = urljoin(self.base_url, relative_url)
    headers = {'x-signature': signature_callback(((full_url + '\n') + json.dumps(payload)))}
    return self.send_request('post', relative_url, payload, headers)
