Add merge_metadata for other ESPs

Support merge_metadata in Mailgun, Mailjet, Mandrill, Postmark, 
SparkPost, and Test backends. (SendGrid covered in earlier PR.)

Also:
* Add `merge_metadata` to AnymailMessage, AnymailMessageMixin
* Add `is_batch()` logic to BasePayload, for consistent handling
* Docs

Note: Mailjet implementation switches *all* batch sending from their 
"Recipients" field to to the "Messages" array bulk sending option.
This allows an independent payload for each batch recipient.
In addition to supporting merge_metadata, this also removes the
prior limitation on mixing Cc/Bcc with merge_data.

Closes #141.

tests/test_general_backend.py

@@ -11,7 +11,7 @@ from django.utils.functional import Promise
 from django.utils.timezone import utc
 from django.utils.translation import ugettext_lazy
 
-from anymail.backends.test import EmailBackend as TestBackend
+from anymail.backends.test import EmailBackend as TestBackend, TestPayload
 from anymail.exceptions import AnymailConfigurationError, AnymailInvalidAddress, AnymailUnsupportedFeature
 from anymail.message import AnymailMessage
 from anymail.utils import get_anymail_setting
@@ -425,3 +425,45 @@ class SpecialHeaderTests(TestBackendTestCase):
         self.message.extra_headers = {"To": "Apparent Recipient <but-not-really@example.com>"}
         with self.assertRaisesMessage(AnymailUnsupportedFeature, "spoofing `To` header"):
             self.message.send()
+
+
+class BatchSendDetectionTestCase(TestBackendTestCase):
+    """Tests shared code to consistently determine whether to use batch send"""
+
+    def setUp(self):
+        super(BatchSendDetectionTestCase, self).setUp()
+        self.backend = TestBackend()
+
+    def test_default_is_not_batch(self):
+        payload = self.backend.build_message_payload(self.message, {})
+        self.assertFalse(payload.is_batch())
+
+    def test_merge_data_implies_batch(self):
+        self.message.merge_data = {}  # *anything* (even empty dict) implies batch
+        payload = self.backend.build_message_payload(self.message, {})
+        self.assertTrue(payload.is_batch())
+
+    def test_merge_metadata_implies_batch(self):
+        self.message.merge_metadata = {}  # *anything* (even empty dict) implies batch
+        payload = self.backend.build_message_payload(self.message, {})
+        self.assertTrue(payload.is_batch())
+
+    def test_merge_global_data_does_not_imply_batch(self):
+        self.message.merge_global_data = {}
+        payload = self.backend.build_message_payload(self.message, {})
+        self.assertFalse(payload.is_batch())
+
+    def test_cannot_call_is_batch_during_init(self):
+        # It's tempting to try to warn about unsupported batch features in setters,
+        # but because of the way payload attrs are processed, it won't work...
+        class ImproperlyImplementedPayload(TestPayload):
+            def set_cc(self, emails):
+                if self.is_batch():  # this won't work here!
+                    self.unsupported_feature("cc with batch send")
+                super(ImproperlyImplementedPayload, self).set_cc(emails)
+
+        connection = mail.get_connection('anymail.backends.test.EmailBackend',
+                                         payload_class=ImproperlyImplementedPayload)
+        with self.assertRaisesMessage(AssertionError,
+                                      "Cannot call is_batch before all attributes processed"):
+            connection.send_messages([self.message])