Added Download / Upload Support

[biocross]

Currently, Swifty only supports URLSessionDataTask, using the .load() methods on NetworkResource. Added support for downloading files using URLSessionDownloadTask, or uploading files, using URLSessionUploadTask would be great!

A simple way to get started on this issue is to add a new property to NetworkResource, which tell it what kind of task it is (Upload/Download/Data), and then add support in SwifyNetworkTask to create the appropriate task. Then a new .download() / .upload() method can be added to the NetworkResource Loader Extensions.

[FlippinBerger]

@biocross mind if I give this a shot?

[biocross]

Sure thing! Go ahead :)