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.
Wow, this sequence is confusing.

You take an id string, make a tag, turn that into a string, parse it into a tag, and take the string of that tag.

Does this seem good to you?

The question is "why is the caller of this function passing an id?"  The only way they would have the id is if they started with the tag.  Lets use the tag.  getOneAction should take a *names.ActionTag.  This method should either take a name.Tag or *names.ActionTag.  Look at where you are calling it from.