Add the option to use path-style S3 URLs.

It's sometimes desirable to use path-style URLs over virtual-host-style URLs. For example, a bucket name with a dot in it will trigger invalid SSL certificate errors on some browsers, since S3's `*.s3.amazonaws.com` certificate only covers single-level subdomains.

This adds the `S3_URL_STYLE` setting, which can be one of `'host'` or `'path'`, defaulting to `'host'` for compatibility.