Add Amazon SNS notification plugin

- Supports a default message or a special message for any of SNS's
protocols (email, sqs, sms, http, https)
  - http://docs.aws.amazon.com/sns/latest/dg/PublishTopic.html

Uses [boto's SNS API](http://boto.readthedocs.org/en/latest/ref/sns.html). For reference, [Amazon's API](http://docs.aws.amazon.com/sns/latest/api/API_Publish.html)