Added default order on purchase object

purchase.py

@@ -96,6 +96,7 @@ class Purchase(OSV):
 
     def __init__(self):
         super(Purchase, self).__init__()
+        self._order[0] = ('id', 'DESC')
         self._error_messages.update({
                 'invoice_addresse_required': 'Invoice addresses must be '
                 'defined for the quotation.',