* 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.
* **Future breaking change:**
Rename all Anymail backends to just `EmailBackend`,
matching Django's naming convention.
(E.g., switch to "anymail.backends.mailgun.EmailBackend"
rather than "anymail.backends.mailgun.MailgunBackend".)
The old names still work, but will issue a DeprecationWarning
and will be removed in some future release.
(Apologies for this change; the old naming convention was
a holdover from Djrill, and I wanted consistency with
other Django EmailBackends before hitting 1.0.)
Fixes#49.
