Added Signature.status(id) method

Provides a new method to get the status of a passed signature using their signatory_id.

It tries to reach the status of an already started signature.

If not, raises an exception

print (self.client.signature.status(signatory_id=555))

{'signatory_email': 'xtorello@gisce.net', 'signatory_status': '6', 'signatory_status_date': '1540284984', 'signatory_id': 555}


print (self.client.signature.status(signatory_id=-1))
NotFoundSignatureException

It also fixes test fixture cassetes to match lates API changes