Background: Applications want to access data in Arvados through a web API. Keep-web currently supports WebDAV. Although it is a standard, 3rd party applications used by customers do not support WebDAV. Instead, many 3rd party applications support reading and writing to S3-compatible object storage. Keep-web should support an S3 compatible API to enable these applications to easily access Arvados. Initial implementation should support the "list objects", "get object", and "put object" APIs using * object name = file path within collection * bucket name = collection UUID * secret key = Arvados API token * endpoint = ExternalURL of keep-web service ...with enough s3 compatibility to test with a common s3 client library like https://github.com/minio/minio-go.