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 on this.  I noticed other query structs have fields named "tag" which are strings.  Is that what you mean?  Or do you want it to contain an ActionTag slice?