Uniter Action(id string) API client/server

This commit adds the ability for Uniter clients to query Actions from
state.State by id.  The response includes a server error message if the
Action is not present on the state server.
That name seemed to me to collide with the State concept of an ActionResult (a thing which is the result sent back after an Action completes), which itself will probably end up being a type in the params package.