Added gzip_zopfli option

Google's Zopfli gzip-compatible compression algorithm takes longer to run but results in a 5% better compression on average.

I think having the option of using Zopfli with s3_website would be a nice feature to have. This pull request makes the necessary changes to require the Zopfli gem and if you include the config parameter "gzip_zopfli: true" then it will use the Zopfli algorithm.

This has previously been requested as an option here: https://github.com/laurilehmijoki/s3_website/issues/49