Adding some early Notifications stubs

* A basic NotificationTemplate model class with early notification type
  definitions
* Initial implementations of the Email, Slack, and Twilio Notification
  backends using the Django email backend system
* Some dependencies thereof

awx/main/notifications/email_backend.py

@@ -0,0 +1,11 @@
+# Copyright (c) 2016 Ansible, Inc.
+# All Rights Reserved.
+
+import logging
+
+from django.core.mail.backends.smtp import EmailBackend
+
+class CustomEmailBackend(EmailBackend):
+
+    init_parameters = ("host", "port", "username", "password",
+                       "use_tls", "use_ssl")