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/mandrill.py

@@ -147,6 +147,24 @@ class MandrillPayload(RequestsPayload):
     def set_track_opens(self, track_opens):
         self.data["message"]["track_opens"] = track_opens
 
+    def set_template_id(self, template_id):
+        self.data["template_name"] = template_id
+        self.data.setdefault("template_content", [])  # Mandrill requires something here
+
+    def set_merge_data(self, merge_data):
+        self.data['message']['preserve_recipients'] = False  # if merge, hide recipients from each other
+        self.data['message']['merge_vars'] = [
+            {'rcpt': rcpt, 'vars': [{'name': key, 'content': rcpt_data[key]}
+                                    for key in sorted(rcpt_data.keys())]}  # sort for testing reproducibility
+            for rcpt, rcpt_data in merge_data.items()
+        ]
+
+    def set_merge_global_data(self, merge_global_data):
+        self.data['message']['global_merge_vars'] = [
+            {'name': var, 'content': value}
+            for var, value in merge_global_data.items()
+        ]
+
     def set_esp_extra(self, extra):
         pass
 
@@ -170,10 +188,7 @@ class MandrillPayload(RequestsPayload):
         ('subaccount', last, None),
         ('google_analytics_domains', last, None),
         ('google_analytics_campaign', last, None),
-        ('global_merge_vars', combine, _expand_merge_vars),
-        ('merge_vars', combine, None),
         ('recipient_metadata', combine, None),
-        ('template_name', last, None),
         ('template_content', combine, _expand_merge_vars),
     )
 
@@ -183,20 +198,9 @@ class MandrillPayload(RequestsPayload):
     def set_ip_pool(self, ip_pool):
         self.data["ip_pool"] = ip_pool
 
-    def set_template_name(self, template_name):
-        self.data["template_name"] = template_name
-        self.data.setdefault("template_content", [])  # Mandrill requires something here
-
     def set_template_content(self, template_content):
         self.data["template_content"] = template_content
 
-    def set_merge_vars(self, merge_vars):
-        # For testing reproducibility, we sort the recipients
-        self.data['message']['merge_vars'] = [
-            {'rcpt': rcpt, 'vars': _expand_merge_vars(merge_vars[rcpt])}
-            for rcpt in sorted(merge_vars.keys())
-        ]
-
     def set_recipient_metadata(self, recipient_metadata):
         # For testing reproducibility, we sort the recipients
         self.data['message']['recipient_metadata'] = [