Added S3DownloadStrategy to download tarballs from private S3 buckets

To use it, add ":using => :s3" to the URL section of your
formula.  This download strategy uses AWS access tokens (in the
environment variables AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY)
to sign the request.  This strategy is good in a corporate setting,
because it lets you use a private S3 bucket as a repo for internal
distribution.  (It will work for public buckets as well.)