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/test.py

@@ -5,13 +5,15 @@ from .base import AnymailBaseBackend, BasePayload
 from ..utils import get_anymail_setting
 
 
-class TestBackend(AnymailBaseBackend):
+class EmailBackend(AnymailBaseBackend):
     """
     Anymail backend that doesn't do anything.
 
     Used for testing Anymail common backend functionality.
     """
 
+    esp_name = "Test"
+
     def __init__(self, *args, **kwargs):
         # Init options from Django settings
         esp_name = self.esp_name
@@ -19,7 +21,7 @@ class TestBackend(AnymailBaseBackend):
                                                   kwargs=kwargs, allow_bare=True)
         self.recorded_send_params = get_anymail_setting('recorded_send_params', default=[],
                                                         esp_name=esp_name, kwargs=kwargs)
-        super(TestBackend, self).__init__(*args, **kwargs)
+        super(EmailBackend, self).__init__(*args, **kwargs)
 
     def build_message_payload(self, message, defaults):
         return TestPayload(backend=self, message=message, defaults=defaults)
@@ -47,6 +49,14 @@ class TestBackend(AnymailBaseBackend):
             raise AnymailAPIError('Unparsable test response')
 
 
+# Pre-v0.8 naming (immediately deprecated for this undocumented test feature)
+class TestBackend(object):
+    def __init__(self, **kwargs):
+        raise NotImplementedError(
+            "Anymail's (undocumented) TestBackend has been renamed to "
+            "'anymail.backends.test.EmailBackend'")
+
+
 class TestPayload(BasePayload):
     # For test purposes, just keep a dict of the params we've received.
     # (This approach is also useful for native API backends -- think of