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/mailgun.py

@@ -162,6 +162,10 @@ class MailgunPayload(RequestsPayload):
             (field, (name, attachment.content, attachment.mimetype))
         )
 
+    def set_envelope_sender(self, email):
+        # Only the domain is used
+        self.sender_domain = email.domain
+
     def set_metadata(self, metadata):
         for key, value in metadata.items():
             self.data["v:%s" % key] = value