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.

README.rst

@@ -43,11 +43,10 @@ built-in `django.core.mail` package. It includes:
 * Simplified inline images for HTML email
 * Normalized sent-message status and tracking notification, by connecting
   your ESP's webhooks to Django signals
+* "Batch transactional" sends using your ESP's merge and template features
 
-Support is planned for:
+Support is also planned for:
 
-* "Bulk-transactional" sends using your ESP's template facility,
-  with portable declaration of substitution/merge data
 * Normalized inbound email processing through your ESP
 
 Anymail is released under the BSD license. It is extensively tested against Django 1.8--1.9