Implements an S3 source.

[cognusion]

The S3 credentials can be provided directly or in the environment. Fully composable and works excellently with the groupcache implementation. Admittedly plays loose with contexts. Resolves #26

In the groupcache example, swapping out testdata and updating newServer() , while taking a new CLI argument for awsS3bucket works perfectly. If the S3 path would be s3://somebucket/food/20200809_0030_01.jpg then http://localhost:8081/food/20200809_0030_01.jpg?&width=1024 would be example path. I can provide a full example if desirable. Happy to iterate on this for a while too if there is feedback.

func newServer() imageserver.Server {
	srv, err := s3.NewS3Server(awsRegion, awsAccessKey, awsSecretKey, awsS3bucket)
	if err != nil {
		panic(err)
	}
	srv = newServerImage(srv)
	srv = newServerGroupcache(srv)
	return srv
}