* 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.
