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/sparkpost.py

@@ -172,6 +172,9 @@ class SparkPostPayload(BasePayload):
             'data': attachment.b64content})
 
     # Anymail-specific payload construction
+    def set_envelope_sender(self, email):
+        self.params['return_path'] = email.addr_spec
+
     def set_metadata(self, metadata):
         self.params['metadata'] = metadata