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/sendgrid_v2.py

@@ -260,7 +260,7 @@ class SendGridPayload(RequestsPayload):
         if files_field in self.files:
             # It's possible SendGrid could actually handle this case (needs testing),
             # but requests doesn't seem to accept a list of tuples for a files field.
-            # (See the MailgunBackend version for a different approach that might work.)
+            # (See the Mailgun EmailBackend version for a different approach that might work.)
             self.unsupported_feature(
                 "multiple attachments with the same filename ('%s')" % filename if filename
                 else "multiple unnamed attachments")