Raise error for invalid/rejected recipients

Raise new MandrillRecipientsRefused exception
when Mandrill returns 'reject' or 'invalid' status
for *all* recipients of a message.

(Similar to Django's SMTP email backend raising
SMTPRecipientsRefused.)

Add setting MANDRILL_IGNORE_RECIPIENT_STATUS
to override the new exception.

Trap JSON parsing errors in Mandrill API response,
and raise MandrillAPIError for them. (Helps with #93.)

Closes #80.
Closes #81.

djrill/tests/mock_backend.py

@@ -7,6 +7,14 @@ from django.test import TestCase
 from django.test.utils import override_settings
 
 
+MANDRILL_SUCCESS_RESPONSE = six.b("""[{
+    "email": "to@example.com",
+    "status": "sent",
+    "_id": "abc123",
+    "reject_reason": null
+}]""")
+
+
 @override_settings(MANDRILL_API_KEY="FAKE_API_KEY_FOR_TESTING",
                    EMAIL_BACKEND="djrill.mail.backends.djrill.DjrillBackend")
 class DjrillBackendMockAPITestCase(TestCase):
@@ -14,7 +22,7 @@ class DjrillBackendMockAPITestCase(TestCase):
 
     class MockResponse(requests.Response):
         """requests.post return value mock sufficient for DjrillBackend"""
-        def __init__(self, status_code=200, raw=six.b("{}"), encoding='utf-8'):
+        def __init__(self, status_code=200, raw=six.b(MANDRILL_SUCCESS_RESPONSE), encoding='utf-8'):
             super(DjrillBackendMockAPITestCase.MockResponse, self).__init__()
             self.status_code = status_code
             self.encoding = encoding