Second attempt to address possible test sequencing issue around tests that alter settings. (Failures in Travis tests not reproducible locally.)
Back-ports override_settings from Django 1.4 for compatibility with Django 1.3.
Add DjrillBackendMockAPITestCase.assert_mandrill_called; use it in representative backend test cases.
(Also make get_api_call_data work with various ways of calling requests.post.)