Allow Cloudfront Invalidations for S3.

Currently, Amazon's Cloudfront servers take up to 24/48 hours to clear through, so you don't see your actual changes online for ages if you overwrite files on Cloudfront.
"Invalidation is a cloudfront feature which removes a distribution object from an edge server cache before the expiration time. It is similar to object expiration. Invalidation request first clear the object from the edge server cache and then request for the object to the origin to fetch the latest version of the object."