title: kivy how to use online images in python
body: <p>I'm trying to make a button which uses an image from URL.</p>
  
  <pre><code>icon= str('https://cdn2.iconfinder.com/data/icons/flat-ui-icons-24-px/24/eye-24-256.png')
  
  self.add_widget(ImageButton(source=(icon), size=(100,100), size_hint=(0.1, 0.1), on_press=callback, pos_hint={"x":0.90, "top":1.0}))
  
  class ImageButton(ButtonBehavior, Image):
      pass
  </code></pre>
  
  <p>It shows as an white image for some reason. Could anyone help me please?</p>