AWS SNS notifier that uses the Java SDK directly

Changed the AWS SNS notifier to use the Java SDK directly.

In case anyone has been using my previous branch, here are the changes you need to make to use the new notifier:

* Credential keywords are now `:access-key` and `:secret-key`, not `:access_key` and `:secret_key`
* Region name now takes a more fully specified form, like `"us-east-1"`, rather than `:us-east`

Also there are new options: `:async`, `:success` and `:error`. When `:async` is set to true , the notifier will use the [async client](http://aws.amazon.com/articles/5496117154196801) under the hood, invoking either `:success` or `:error` callback when the request has completed.