title: Allow only positive decimal numbers
body: <p>Within my Django models I have created a decimal field like this:</p>
  
  <pre><code>price = models.DecimalField(_(u'Price'), decimal_places=2, max_digits=12)
  </code></pre>
  
  <p>Obviously it makes no sense for the price to be negative or zero.  Is there a way to limit the decimal number to only positive numbers?  </p>
  
  <p>Or do I have to capture this using form validation?</p>