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.
You could restructure this slightly:
```go 
func (u *UniterAPI) getOneActionById(actionId string) (params.ActionsQueryResult, error) {
	result := params.ActionsQueryResult{}
	action, err := u.st.Action(actionId)
	if err != nil {
		return result, err
	}

	result.Action = &params.Action{
		Name:   action.Name(),
		Params: action.Payload(),
	}
	return result, nil
}
```