Clean up and document Anymail's test EmailBackend

* Change Anymail's test EmailBackend to collect sent messages in
  django.core.mail.outbox, same as Django's own locmem EmailBackend.
  (So Django's test runner will automatically clear accumulated mail
  between test cases.)

* Rename EmailMessage `test_response` attr to `anymail_test_response`
  to avoid conflicts, and record merged ESP send params in
  new `anymail_send_params` attr.

* Add docs

Closes #36.

docs/tips/index.rst

@@ -10,6 +10,7 @@ done with Anymail:
    multiple_backends
    django_templates
    securing_webhooks
+   test_backend
 
 .. TODO:
 ..    Working with django-mailer(2)