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 sure I see the problem.  Either there's lots of precedent or I'm misunderstanding you: 
 - https://github.com/juju/juju/blob/master/state/api/params/internal.go#L233-L243
 - https://github.com/juju/juju/blob/master/state/api/params/internal.go#L280-L287
 - https://github.com/juju/juju/blob/master/state/api/params/internal.go#L402-L409
 - https://github.com/juju/juju/blob/master/state/api/params/internal.go#L424-L429