Add property for numeric address code

This probably should be done with a migration to change the field type
of the existing numeric field, and to correct all values to use the
3-digit form.  However, that would be quite painful, so I have
shamefully opted for the easy way out: provide a simple class property
to format the number in the right way.

oscar/apps/address/abstract_models.py

                                      primary_key=True)
     iso_3166_1_a3 = models.CharField(_('ISO 3166-1 alpha-3'), max_length=3,
                                      null=True, db_index=True)
+    # This should have been a CharField as it needs to be padded with zeros to
+    # be 3 digits.  Access via the numeric_code instead.
     iso_3166_1_numeric = models.PositiveSmallIntegerField(
         _('ISO 3166-1 numeric'), null=True, db_index=True)
     name = models.CharField(_('Official name (CAPS)'), max_length=128)
     def __unicode__(self):
         return self.printable_name or self.name
 
+    @property
+    def numeric_code(self):
+        return u"%.03d" % self.iso_3166_1_numeric
+
 
 class AbstractShippingAddress(AbstractAddress):
     """