Allow kwargs overrides for (nearly) all settings

* Update utils.get_anymail_setting to support
  kwargs override of django.conf.settings values
* Use the updated version everywhere
* Switch from ImproperlyConfigured to
  AnymailConfigurationError exception
  (anticipates feature_wehooks change)

Closes #12

tests/test_general_backend.py

@@ -0,0 +1,35 @@
+from django.core.mail import get_connection
+from django.test import SimpleTestCase
+from django.test.utils import override_settings
+
+from .utils import AnymailTestMixin
+
+
+class BackendSettingsTests(SimpleTestCase, AnymailTestMixin):
+    """Test settings initializations for Anymail EmailBackends"""
+
+    # We should add a "GenericBackend" or something basic for testing.
+    # For now, we just access real backends directly.
+
+    @override_settings(ANYMAIL={'MAILGUN_API_KEY': 'api_key_from_settings'})
+    def test_connection_kwargs_overrides_settings(self):
+        connection = get_connection('anymail.backends.mailgun.MailgunBackend')
+        self.assertEqual(connection.api_key, 'api_key_from_settings')
+
+        connection = get_connection('anymail.backends.mailgun.MailgunBackend',
+                                    api_key='api_key_from_kwargs')
+        self.assertEqual(connection.api_key, 'api_key_from_kwargs')
+
+    @override_settings(ANYMAIL={'SENDGRID_USERNAME': 'username_from_settings',
+                                'SENDGRID_PASSWORD': 'password_from_settings'})
+    def test_username_password_kwargs_overrides(self):
+        # Additional checks for username and password, which are special-cased
+        # because of Django core mail function defaults.
+        connection = get_connection('anymail.backends.sendgrid.SendGridBackend')
+        self.assertEqual(connection.username, 'username_from_settings')
+        self.assertEqual(connection.password, 'password_from_settings')
+
+        connection = get_connection('anymail.backends.sendgrid.SendGridBackend',
+                                    username='username_from_kwargs', password='password_from_kwargs')
+        self.assertEqual(connection.username, 'username_from_kwargs')
+        self.assertEqual(connection.password, 'password_from_kwargs')