Add notify models + context from original project

[areski]

The original project stored notification and had few features important for this project.
https://github.com/Star2Billing/django-notification/tree/master/notification

This PR is to open a discussion how to add them back properly.

[nemesifier]

I was expecting to find this feature in this app.

[KatherineMichel]

@nemesisdesign We are going to be going through all of the outstanding PRs as part of a new major release of Pinax. Sorry for any inconvenience.