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/message.py

@@ -25,6 +25,9 @@ class AnymailMessageMixin(object):
         self.tags = kwargs.pop('tags', UNSET)
         self.track_clicks = kwargs.pop('track_clicks', UNSET)
         self.track_opens = kwargs.pop('track_opens', UNSET)
+        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.anymail_status = None
 
         # noinspection PyArgumentList