This should not directly affect more complex apps that don't use the ASBasicImageDownloader.
Also disables the default-on text placeholders, as they churn memory during the measurement pass.
These were intended to be written with pure layers (without using backing stores), so I don't
think it is a reasonable default-on behavior until that is fixed.
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.)
