S3 server-side encryption based on ENV["PGBACKUPS_SERVER_SIDE_ENCRYPTION...

Great gem; exactly what I was looking for and more. Thank you!

I added the ability to take advantage of [AWS S3 Server-Side Encryption](https://aws.amazon.com/blogs/aws/new-amazon-s3-server-side-encryption/) by setting 

    ENV["PGBACKUPS_SERVER_SIDE_ENCRYPTION"] = true

[Fog supports Server-Side Encryption](http://rubydoc.info/gems/fog/Fog/Storage/AWS/File#save-instance_method) via the ``{ :encryption => "AES256" }`` option.