This creates an EmailList object that presents a simple API to use in controllers.

I just wanted to abstract away the Mailchimp service in case we end up switching services someday. This class gives a nice API to use in the controller.

```
EmailList.announcements.subscribe("jonmagic@gmail.com")
```

@mikefowler let me know what you think, I'll add tests and actually make it all work if you like the approach.