Fix for issue #3: If Tweet text contains only a URL, show the URL

The "url" for newer tweets look like "http://t.co/2Wbsm7IIt7", so it may be better one of these: 
          "expanded_url": "http://www.google.com",
          "display_url": "google.com".
Merge branch 'patch-2' of github.com:hugovk/twelescreen into hugovk-patch-2
If a Tweet contains only a URL, display it.