ASBasicImageDownloader.

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.)
Convert Kittens sample project to ASNetworkImageNode.

Working-range-driven image loading.

(Note that ASNetworkImageNode and ASMultiplexImageNode haven't yet been
updated to work with the new `-reclaimMemory`-based working range
system, so this doesn't quite behave correctly yet.)