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.

anymail/backends/test.py

@@ -22,6 +22,9 @@ class EmailBackend(AnymailBaseBackend):
     esp_name = "Test"
 
     def __init__(self, *args, **kwargs):
+        # Allow replacing the payload, for testing.
+        # (Real backends would generally not implement this option.)
+        self._payload_class = kwargs.pop('payload_class', TestPayload)
         super(EmailBackend, self).__init__(*args, **kwargs)
         if not hasattr(mail, 'outbox'):
             mail.outbox = []  # see django.core.mail.backends.locmem
@@ -32,7 +35,7 @@ class EmailBackend(AnymailBaseBackend):
         return mail.outbox.index(message)
 
     def build_message_payload(self, message, defaults):
-        return TestPayload(backend=self, message=message, defaults=defaults)
+        return self._payload_class(backend=self, message=message, defaults=defaults)
 
     def post_to_esp(self, payload, message):
         # Keep track of the sent messages and params (for test cases)
@@ -130,6 +133,9 @@ class TestPayload(BasePayload):
     def set_merge_data(self, merge_data):
         self.params['merge_data'] = merge_data
 
+    def set_merge_metadata(self, merge_metadata):
+        self.params['merge_metadata'] = merge_metadata
+
     def set_merge_global_data(self, merge_global_data):
         self.params['merge_global_data'] = merge_global_data