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/backends/base.py

@@ -190,9 +190,12 @@ class AnymailBaseBackend(BaseEmailBackend):
         """
         Read-only name of the ESP for this backend.
 
-        (E.g., MailgunBackend will return "Mailgun")
+        Concrete backends must override with class attr. E.g.:
+            esp_name = "Postmark"
+            esp_name = "SendGrid"  # (use ESP's preferred capitalization)
         """
-        return self.__class__.__name__.replace("Backend", "")
+        raise NotImplementedError("%s.%s must declare esp_name class attr" %
+                                  (self.__class__.__module__, self.__class__.__name__))
 
 
 class BasePayload(object):