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 would expect the usual pattern:

    type Action struct {
        Name string `please make json serialization explicit`
        Params string `explicit again please`
    }

    type ActionResult struct {
        Result *Action `explicit again, I'll stop saying it now`
        Error *Error
    }

    type ActionResults struct {
        Results []ActionResult
    }