Feature: Add envelope_sender

New EmailMessage attribute `envelope_sender` controls ESP's sender,
sending domain, or return path where supported:

* Mailgun: overrides SENDER_DOMAIN on individual message
  (domain portion only)
* Mailjet: becomes `Sender` API param
* Mandrill: becomes `return_path_domain` API param
  (domain portion only)
* SparkPost: becomes `return_path` API param
* Other ESPs: not believed to be supported

Also support undocumented Django SMTP backend behavior, where envelope
sender is given by `message.from_email` when
`message.extra_headers["From"]` is set. Fixes #91.

anymail/backends/test.py

@@ -74,6 +74,9 @@ class TestPayload(BasePayload):
     def set_from_email(self, email):
         self.params['from'] = email
 
+    def set_envelope_sender(self, email):
+        self.params['envelope_sender'] = email.addr_spec
+
     def set_to(self, emails):
         self.params['to'] = emails
         self.recipient_emails += [email.addr_spec for email in emails]