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.
Here we have something called UnitId going in to an argument called unitTag. Tags and Ids are different things in Juju-land (ids are typically contained within tags) so please make this consistent with what the value actually represents.