Un-hardcode message_id in test backend; add console backend

* Un-hardcode status message_id in test backend

For the test EmailBackend, get message ID's based on array position in
`mail.outbox`, so that tests can predict the message ID.


* Add a console backend for use in development

Adds an EmailBackend derived from both Anymail's test backend and
Django's console backend, to provide anymail statuses and signal
handling while printing messages to the console.  For use during
development on localhost.

Closes #87

anymail/backends/test.py

@@ -27,6 +27,11 @@ class EmailBackend(AnymailBaseBackend):
         if not hasattr(mail, 'outbox'):
             mail.outbox = []  # see django.core.mail.backends.locmem
 
+    def get_esp_message_id(self, message):
+        # Get a unique ID for the message.  The message must have been added to
+        # the outbox first.
+        return mail.outbox.index(message)
+
     def build_message_payload(self, message, defaults):
         return TestPayload(backend=self, message=message, defaults=defaults)
 
@@ -41,7 +46,10 @@ class EmailBackend(AnymailBaseBackend):
                 raise response
         except AttributeError:
             # Default is to return 'sent' for each recipient
-            status = AnymailRecipientStatus(message_id=1, status='sent')
+            status = AnymailRecipientStatus(
+                message_id=self.get_esp_message_id(message),
+                status='sent'
+            )
             response = {
                 'recipient_status': {email: status for email in payload.recipient_emails}
             }