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.
I'm not really clear why we need a batch call and this degree of type abstraction when Actions will only ever be assembled one at a time by the RunHook method when it receives an Action off the uniter channel.