Add event unwrapper: s3-in-sns

[jthomerson]

A common pattern in Lambda functions is to be subscribed to an SNS topic
that is sending out S3 notification events. Getting down to the meat of
the S3 event(s) that triggered the Lambda function requires quite a bit
of parsing through the event structure(s), so this unwrapper makes that
into a reusable function.

[coveralls]

Coverage remained the same at 100.0% when pulling 0be0aba on add_event_unwrapper_s3-in-sns into 2d0590f on master.