Add aws sns notifier

Adds support for sending events to Amazon Web Service's [Simple Notification Service](http://aws.amazon.com/sns/).

Let me know if any of the changes do not conform to the project's/clojure's coding conventions. I'm especially dubious about moving `riemann.email/subject` and `riemann.email/body` to `riemann.common`: they're surely being shared between `riemann.email` and `riemann.sns` but may not be common enough to warrant a seat in `riemann.common`, depending on your preferences.