Add ESP templates, batch send and merge

* message.template_id to use ESP stored templates
* message.merge_data and merge_global_data
  to supply per-recipient/global merge variables
  (with or without an ESP stored template)
* When using per-recipient merge_data, tell ESP to use
  batch send: individual message per "to" address.
  (Mailgun does this automatically; SendGrid requires
  using a different "to" field; Mandrill requires
  `preserve_recipients=False`; Postmark doesn't
  support *this type* of batch sending with merge data.)
* Allow message.from_email=None (must be set after
  init) and message.subject=None to suppress those
  fields in API calls (for ESPs that allow "From" and
  "Subject" in their template definitions).

Mailgun:
* Emulate merge_global_data by copying to
  recipient-variables for each recipient.

SendGrid:
* Add delimiters to merge field names via
  esp_extra['merge_field_format'] or
  ANYMAIL_SENDGRID_MERGE_FIELD_FORMAT setting.

Mandrill:
* Remove Djrill versions of these features;
  update migration notes.

Closes #5.

tests/test_postmark_backend.py

@@ -332,6 +332,34 @@ class PostmarkBackendAnymailFeatureTests(PostmarkBackendMockAPITestCase):
         with self.assertRaisesMessage(AnymailUnsupportedFeature, 'track_clicks'):
             self.message.send()
 
+    def test_template(self):
+        self.message.template_id = 1234567
+        # Postmark doesn't support per-recipient merge_data
+        self.message.merge_global_data = {'name': "Alice", 'group': "Developers"}
+        self.message.send()
+        self.assert_esp_called('/email/withTemplate/')
+        data = self.get_api_call_json()
+        self.assertEqual(data['TemplateId'], 1234567)
+        self.assertEqual(data['TemplateModel'], {'name': "Alice", 'group': "Developers"})
+
+    def test_merge_data(self):
+        self.message.merge_data = {
+            'alice@example.com': {'name': "Alice", 'group': "Developers"},
+        }
+        with self.assertRaisesMessage(AnymailUnsupportedFeature, 'merge_data'):
+            self.message.send()
+
+    def test_missing_subject(self):
+        """Make sure a missing subject omits Subject from API call.
+
+        (Allows use of template subject)
+        """
+        self.message.template_id = 1234567
+        self.message.subject = None
+        self.message.send()
+        data = self.get_api_call_json()
+        self.assertNotIn('Subject', data)
+
     def test_default_omits_options(self):
         """Make sure by default we don't send any ESP-specific options.
 
@@ -342,6 +370,8 @@ class PostmarkBackendAnymailFeatureTests(PostmarkBackendMockAPITestCase):
         self.message.send()
         data = self.get_api_call_json()
         self.assertNotIn('Tag', data)
+        self.assertNotIn('TemplateId', data)
+        self.assertNotIn('TemplateModel', data)
         self.assertNotIn('TrackOpens', data)
 
     def test_esp_extra(self):