Support Postmark delivery event webhook.

Add support for Postmark's recently-released [delivery tracking webhook] to Anymail's normailized status event handling. The existing Anymail tracking webhook URL can be copied to "Delivery webhook" in your Postmark outbound server settings.

Closes #45.

tests/test_postmark_webhooks.py

@@ -55,6 +55,30 @@ class PostmarkDeliveryTestCase(WebhookTestCase):
         self.assertEqual(event.mta_response,
                          "smtp;550 5.1.1 The email account that you tried to reach does not exist.")
 
+    def test_delivered_event(self):
+        raw_event = {
+            "ServerId": 23,
+            "MessageID": "883953f4-6105-42a2-a16a-77a8eac79483",
+            "Recipient": "recipient@example.com",
+            "Tag": "welcome-email",
+            "DeliveredAt": "2014-08-01T13:28:10.2735393-04:00",
+            "Details": "Test delivery webhook details"
+        }
+        response = self.client.post('/anymail/postmark/tracking/',
+                                    content_type='application/json', data=json.dumps(raw_event))
+        self.assertEqual(response.status_code, 200)
+        kwargs = self.assert_handler_called_once_with(self.tracking_handler, sender=PostmarkTrackingWebhookView,
+                                                      event=ANY, esp_name='Postmark')
+        event = kwargs['event']
+        self.assertIsInstance(event, AnymailTrackingEvent)
+        self.assertEqual(event.event_type, "delivered")
+        self.assertEqual(event.esp_event, raw_event)
+        self.assertEqual(event.timestamp, datetime(2014, 8, 1, 13, 28, 10, microsecond=273539,
+                                                   tzinfo=get_fixed_timezone(-4*60)))
+        self.assertEqual(event.message_id, "883953f4-6105-42a2-a16a-77a8eac79483")
+        self.assertEqual(event.recipient, "recipient@example.com")
+        self.assertEqual(event.tags, ["welcome-email"])
+
     def test_open_event(self):
         raw_event = {
             "FirstOpen": True,