S3 friendly proxy

At the moment, s3 does not use proxy defined in the environment i.e. using HTTP_PROXY. This adds `Proxy: http.ProxyFromEnvironment` to the Transport so that it will use the environment variables if they are defined. Without this, it is not possible to access s3 through a proxy. I don't really know how to add a test for that. Let me know if this is ok or if you want me to submit a new PR with the commits squashed.