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.
This was discussed with @bz2 and @fwereade -- 

The StringsWatcher watching the Actions collection on State responds with Ids at this time, so we're constructing our HookInfo's with Ids (this happens internally to the client.)  At the time the Action is fetched from the stateserver, the Id is sent to the query point on the client API.  This method then turns it into a Tag for the query proper.

I received really clear direction that we're not to muddle about with Tags on the client side, only over the wire, so this is what made sense to me given that understanding.  Very open to hearing otherwise but I think we need to get on the same page altogether here.  Also @johnweldon fwiw