support private s3:// urls for rubygems repos
implement s3 signing in rubygems itself

Depending on the aws-sdk gem for something as core as rubygems smelt
funny. Fortunately signing GET requests is pretty easy.
ruby 1.8.7 doesn't have Base64.strict_encode64
Merge pull request #856 from codekitchen/s3_fetch

RemoteFetcher private S3 repo support