Models

@wesleykendall This pull request includes the models for `entity_subscription` and a basic readme of the semantics of the models.

I tried to be very careful in the readme about explaining the difference between "source" and "medium" both from the perspective of a user receiving notifications, and from an application developer creating/sending notifications. Let me know if the readme is clear on that front. I think maintaining this distinction is key to keeping a flexible subscriptions app.

This pull request does not include any of the rest of the API for working with subscriptions. Those functions will be developed in tandem with emails and notifications.
In setup.py there is a field called `long_description` that is used as the text displayed on the pypi page for that project. It expects something in ReStructuredText format. Since we just use the readme as our `long_description` I've been making my public projects use RST for their readmes (github handles it fine). 

The difference is a pypi page that looks like this: https://pypi.python.org/pypi/django-localized-recurrence/0.2.1 as opposed to this: https://pypi.python.org/pypi/django-entity/0.4.2