sns: use hyphen instead of underscore in symbols to match the project's naming conventions
sns: improve test organization: extract fake args, make aws region configurable
sns: extract function compose-message and add test for body truncation
sns: use vanilla AWS SDK. also add async support
sns: support hardcoding of subject and body as demonstrated in the doc string
Merge pull request #199 from glucoseinc/add-vanilla-aws-sns-notifier

AWS SNS notifier that uses the Java SDK directly