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.

anymail/backends/mailgun.py

@@ -56,6 +56,12 @@ class MailgunPayload(RequestsPayload):
         auth = ("api", backend.api_key)
         self.sender_domain = None
         self.all_recipients = []  # used for backend.parse_recipient_status
+
+        # late-binding of recipient-variables:
+        self.merge_data = None
+        self.merge_global_data = None
+        self.to_emails = []
+
         super(MailgunPayload, self).__init__(message, defaults, backend, auth=auth, *args, **kwargs)
 
     def get_api_endpoint(self):
@@ -66,6 +72,34 @@ class MailgunPayload(RequestsPayload):
                                backend=self.backend, email_message=self.message, payload=self)
         return "%s/messages" % self.sender_domain
 
+    def serialize_data(self):
+        self.populate_recipient_variables()
+        return self.data
+
+    def populate_recipient_variables(self):
+        """Populate Mailgun recipient-variables header from merge data"""
+        merge_data = self.merge_data
+
+        if self.merge_global_data is not None:
+            # Mailgun doesn't support global variables.
+            # We emulate them by populating recipient-variables for all recipients.
+            if merge_data is not None:
+                merge_data = merge_data.copy()  # don't modify the original, which doesn't belong to us
+            else:
+                merge_data = {}
+            for email in self.to_emails:
+                try:
+                    recipient_data = merge_data[email]
+                except KeyError:
+                    merge_data[email] = self.merge_global_data
+                else:
+                    # Merge globals (recipient_data wins in conflict)
+                    merge_data[email] = self.merge_global_data.copy()
+                    merge_data[email].update(recipient_data)
+
+        if merge_data is not None:
+            self.data['recipient-variables'] = self.serialize_json(merge_data)
+
     #
     # Payload construction
     #
@@ -87,8 +121,10 @@ class MailgunPayload(RequestsPayload):
     def set_recipients(self, recipient_type, emails):
         assert recipient_type in ["to", "cc", "bcc"]
         if emails:
-            self.data[recipient_type] = [str(email) for email in emails]
+            self.data[recipient_type] = [email.address for email in emails]
             self.all_recipients += emails  # used for backend.parse_recipient_status
+        if recipient_type == 'to':
+            self.to_emails = [email.email for email in emails]  # used for populate_recipient_variables
 
     def set_subject(self, subject):
         self.data["subject"] = subject
@@ -145,6 +181,17 @@ class MailgunPayload(RequestsPayload):
     def set_track_opens(self, track_opens):
         self.data["o:tracking-opens"] = "yes" if track_opens else "no"
 
+    # template_id: Mailgun doesn't offer stored templates.
+    # (The message body and other fields *are* the template content.)
+
+    def set_merge_data(self, merge_data):
+        # Processed at serialization time (to allow merging global data)
+        self.merge_data = merge_data
+
+    def set_merge_global_data(self, merge_global_data):
+        # Processed at serialization time (to allow merging global data)
+        self.merge_global_data = merge_global_data
+
     def set_esp_extra(self, extra):
         self.data.update(extra)
         # Allow override of sender_domain via esp_extra