ASBasicImageDownloader iOS 7 support

On iOS 7, NSURLSessionTask is actually a [__NSCFURLSessionTask](https://github.com/EthanArbuckle/IOS-7-Headers/blob/master/Frameworks/CFNetwork.framework/__NSCFURLSessionTask.h) (a private class) and our NSURLSessionTask category to add the associated object asyncdisplaykit_metadata was crashing on iOS 7 stating that there was no method. This commit changes the category to extend NSURLRequest as well as change NSURLSessionTask to NSURLSessionDownloadTask.
This assert was failing on iOS 7 w/ NSURLSessionTask, but it works with NSURLSessionDownloadTask, which is just a subclass of NSURLSessionTask. 

¯\_(ツ)_/¯