execute JSON-encoded regression tests against multiple backends

This is looking much better after the third re-write or so. Now using Argonaut's DecodeJson and a RegressionTest case class to encapsulate loading the JSON tests, and using Tasks in a fairly disciplined way to encapsulate/chain the effects of setup, query, and verifying results.
This is a raw effect (as in several places). It's best to compose Tasks and run the effect at the last possible moment that Specs forces you to run it.