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/message.py

@@ -29,6 +29,7 @@ class AnymailMessageMixin(object):
         self.template_id = kwargs.pop('template_id', UNSET)
         self.merge_data = kwargs.pop('merge_data', UNSET)
         self.merge_global_data = kwargs.pop('merge_global_data', UNSET)
+        self.merge_metadata = kwargs.pop('merge_metadata', UNSET)
         self.anymail_status = AnymailStatus()
 
         # noinspection PyArgumentList