Conditionally include a 'guid' property in info returned by validator.

The only situation this doesn't work for is when a raw 0.5.0-style
assertion object is passed into the validator: since there's no way
to know the URL at which the object is hosted, there's no way to
provide a GUID.
Merge pull request #12 from toolness/include-guid-in-info-3

Conditionally include a 'guid' property in info returned by validator.