Added AWS4DownloadStrategy, which downloads tarballs from public and private S3 buckets

AWS4DownloadStrategy uses your AWS access tokens (in the standard 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.  (AWS S3 = AWS4, get it!)