Rename EmailBackends for Django consistency

* **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.

anymail/exceptions.py

@@ -175,3 +175,7 @@ class AnymailWarning(Warning):
 
 class AnymailInsecureWebhookWarning(AnymailWarning):
     """Warns when webhook configured without any validation"""
+
+
+class AnymailDeprecationWarning(AnymailWarning, DeprecationWarning):
+    """Warning for deprecated Anymail features"""