S3 logging

Somewhat separately (but there's more to see), I've added s3 logging to the fastly gem too. However, it does change a little when it's merged with the other branch I've submitted a PR for - until it's merged with that branch, list_s3loggings doesn't work (along with all the other list_* methods), and after it's merged it changes from get_s3logging, list_s3loggings etc. to get_s3_logging, list_s3_logging, etc.

If it would be easier I can pull this back and resubmit it after such time that other PR gets accepted, but it's different enough I decided to submit two PRs.