Refs #65 — Make sure we can verify only one MSISDN with an HawkSession.
Refs #65 — Add tests for nexmo_callback and multiple number validation
Tarek review.
Merge pull request #65 from mozilla-services/65-one-msisdn-per-session

Make sure we enforce only one MSISDN validation per hawk session
Fix #73 — Return the moVerificer MCC/MNC specific number.