NSDataWithBlocks: NSData Category for iOS
Currently, NSData:dataWithContentsOfURL: is a blocking request - meaning it 
blocks the main thread until the contents are downloaded. The purpose of this 
code is to wrap NSData:dataWithContentsOfURL: in a block to create a non-blocking call.

Some of the other frameworks provide a this capability, but I didn't need the everything
else the frameworks were providing.

Feel free to use any way you want. Or better yet, add to it!