added aws-managed-encryption option for AES256 issue #33, fixed README #30

added aws-managed-encryption option for AES256 issue #33. This forces all uploads to set the 'x-amz-server-side​-encryption' option as documented here:  http://docs.aws.amazon.com/AmazonS3/latest/dev/SSEUsingRESTAPI.html

minor README edit for recheck-s3 option