title: Work out correct money denominations for change
body: <p>Working on a django project.
  On my payment model I have a simple def save</p>
  
  <pre><code>def save(self, *args, **kwargs):
      self.amount_change = self.amount_due - self.amount_paid
      return super(Payment, self).save(*args, **kwargs)
  </code></pre>
  
  <p>If my amount_change comes to <code>-455.50</code> I'd like to return change as</p>
  
  <ul>
  <li>2x200   </li>
  <li>1x50   </li>
  <li>1x5   </li>
  <li>1x0.5</li>
  </ul>
  
  <p>What I'd like to do is breakdown the amount_change into the money denominations that I have and return the change to the client with the correct notes and or coins.
  My denominations are [200, 100, 50, 20, 10, 5, 1, 0.5]</p>
  
  <p>How do I go about doing this?
  Any help is appreciated.</p>