'Attempt to authenticate for a particular cosigner.
:param id: the id of the cosigner
:param transaction: the hex encoded [partially signed] compact transaction to sign
:param otp: the one time password'

def sign(self, id, transaction, otp):

    payload = {'otp': otp, 'transaction': transaction}
    return self.send_request('post', ('cosigner/%s/sign' % quote(id)), payload)
