Adds protocol relative URL support to the S3Link view helper.

Protocol relative URLs (e.g. "//my-bucket.s3.amazonaws.com/my-object") are useful to make an efficient use of your S3 resources; on secure pages, secure objects will be requested and on insecure pages, insecure objects will be requested.

Backwards compatibility is maintained in this patch, with HTTPS links remaining the default and the setUseSsl() and getUseSsl() functions remaining.  I have marked them deprecated as using ssl or not is no longer a binary decision after this patch, as three schemes (https, http and protocol relative) are now supported and so setScheme() and getScheme() are more appropriate.