title: Execute swift nsurlsession Tasks in serial order
body: <p>I don’t know how to execute multiple NSUrlSession tasks one by one. When first task finishes start second task, when second task finishes start third task etc</p>
  
  <p>For example I want to download multiple files from a web service.</p>
  
  <p>I want them to begin downloading one by one.</p>
  
  <p>For example files: 1.png , 2.png , 3.png</p>
  
  <p>I want them to be downloaded in the same order I wrote them. </p>
  
  <p>How can I do that with NSUrlSession?</p>