Unsigned request option

The AWS CLI has a [`--no-sign-request`](https://github.com/aws/aws-cli/commit/64822485f48d2d0f7916d957ae5fd9e4e68cd907) option which I've found useful for anonymously downloading S3 objects with `public-read` ACLs, and I'd like to see a similar feature added to the Ruby SDK.

Although it's currently possible to pass an S3 object's `#public_url` to [`open`](http://ruby-doc.org/stdlib-2.3.0/libdoc/open-uri/rdoc/OpenURI.html) or another Ruby HTTP client, I'd like to be able to leverage the AWS SDK's existing functionality for optimized downloading, file integrity verification and network-error retries, but without requiring credentials to be provided when not necessary.