Bring back this convenience API -- it disappeared somewhere along the
line while we were building Paper. This is totally trivial, but
conveniently won't break if you layer-back a leaf node.
Closes#278.
This is a simplistic, NSURLSession-based downloader object that
implements ASImageDownloaderProtocol and can be used with
ASMultiplexImageNode and ASNetworkImageNode.
(Closes#115. NSURLSession should suffice for most usecases, and this
code should provide a good jumping-off point for a more-complex
implementation.)
