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/base.py

@@ -197,6 +197,9 @@ class BasePayload(object):
         ('tags', combine, None),
         ('track_clicks', last, None),
         ('track_opens', last, None),
+        ('template_id', last, None),
+        ('merge_data', combine, None),
+        ('merge_global_data', combine, None),
         ('esp_extra', combine, None),
     )
     esp_message_attrs = ()  # subclasses can override
@@ -356,6 +359,15 @@ class BasePayload(object):
     def set_track_opens(self, track_opens):
         self.unsupported_feature("track_opens")
 
+    def set_template_id(self, template_id):
+        self.unsupported_feature("template_id")
+
+    def set_merge_data(self, merge_data):
+        self.unsupported_feature("merge_data")
+
+    def set_merge_global_data(self, merge_global_data):
+        self.unsupported_feature("merge_global_data")
+
     # ESP-specific payload construction
     def set_esp_extra(self, extra):
         self.unsupported_feature("esp_extra")