Add GetTorrent and GetTorrentReader

* Turns most of `GetResponse` into `getResponseParams`, which lets you pass in extra URL params -- I didn't really like exposing a new top level get with params, but that would be functional for my purposes if you'd rather do that.
* Use `getResponseParams` to power `GetTorrent*` functions, by passing in `?torrent` to get a Torrent file.

http://docs.aws.amazon.com/AmazonS3/latest/API/RESTObjectGETtorrent.html