Provide display-name for title/name-free objects.

Per [71833168](https://www.pivotaltracker.com/story/show/71833168).  Was showing "undefined" because they (Requests/Responses) lack the name or title required for the `display_name` method.  Considered making it show the "hidden" title that is created, but the user may not recognize this.  Solution is to extend the display_name method so that it falls back to referring to the object by its (truncated) description.