Add expiration check to verification

[kimdhamilton]

See 6722a8660ef5d00cec60c1a41a53ffa552111f41 of cert-verifier. This field is optional.

Expires field comes from Open Badges. Type is DateTime, defined as follows:

DateTime - Either an ISO 8601 date or a standard 10-digit Unix timestamp.

Note that, if it is a ISO 8601 date string:

• time component is optional
• timezone info is optional. If not included, assume UTC

[kimdhamilton]

This is a good candidate for a starter task. See http://www.blockcerts.org/guide/verification-process.html and implementation in cert-verifier