Policy eval generification

[rnorton5432]

This PR is mostly deletions following off of #30.

I have removed most of the policy-specific result types like GetPackagesResult and replaced them with generic query methods. Each downstream policy repository should define its own queries and query types to match its needs.

The split of async/subscription/local skill handlers have been merged into a single one. This should reduce effort required when a change needs to affect all of them. Each capability is now selected as an option when creating the handler. Depending on which options are enabled, things like where the subscription data comes from or whether or not to transact are determined via hooks into the main flow.

As a demonstration of what this PR would look like when consumed, the "No Stale Base Images" and "Approved Base Images" policies have been converted at atomist-skills/policy-base-images.

A template repository which can be used to generate new policy repositories is at atomist-skills/template-go-policy.

[chrispatrick]

Looking pretty good! Just added a handful of comments 🙂