title: Django CommaSeperatedIntegerField and negative numbers
body: <p>If I had the following class:</p>
  
  <pre><code>class foo(models.Model):
      list = models.CommaSeparatedIntegerField(max_length=255)
  </code></pre>
  
  <p>If I try to add a foo object from the admin, it will not let me populate list with negative numbers. For instance, if I populate the field with 1,2,3,4 its fine. But 1,2,3,-4 will give me the error message "Enter only digits separated by commas." Any ideas?</p>
  
  <p>Thanks!</p>