Allow a custom SSL CA to be specified for S3 connections

Opening early in hopes of discussing this.

We want to use goamz with an internal Ceph server's S3 API, which we are using with SSL certificates issued by an internal CA. We would rather not install these certs into the system root, as they should only be trusted for certain connections. This PR represents the smallest patch I could concoct to allow this to work. It would require a user discretely call the SetSSLCACertificate() method on the s3 object.

Please feel free to provide any feedback on this, including how you'd like to see it tested.