title: Copy a part of NSData byte array to another NSData type
body: <p>I have an original <code>NSData</code> type which contains let's say 100 bytes. I want to get 2 other <code>NSData</code> types. The first containing the first 20 bytes of the 100, and the second one containing the other 80.
  They should be copied from the original <code>NSData</code>. Sorry if I wasn't so clear, but I'm pretty new with Objective-C.</p>