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
Add some failure handling for `region` and `publish`
Merge pull request #6740 from mjschultz/add-sns-notification

Add Amazon SNS notification plugin