title: Swift 4 - Notification Center addObserver issue
body: <p>I'm crashing and getting an <code>unrecognized selector</code> error every time a <code>Notification</code> arrives and the App tries to execute its associated method.
  Here's my code - which is in <code>viewDidLoad</code>:</p>
  
  <pre><code>let notificationCenter = NotificationCenter.default
  notificationCenter.addObserver(self, selector: Selector(("sayHello")), name:NSNotification.Name(rawValue: "dataDownloadCompleted"), object: nil)
  </code></pre>
  
  <p>The <code>sayHello()</code> method is quite simple - looks like this:</p>
  
  <pre><code>func sayHello() {
      print("Hello")
  }
  </code></pre>
  
  <p>I've verified that the <code>Notification</code> is posted successfully and that it <em>arrives</em> successfully - so that's not the issue. The crash happens when the App looks to act upon the arrival of the <code>Notification</code> - by executing the <code>sayHello()</code> method. It keeps giving me that <code>unrecognized selector</code> error.</p>
  
  <p>Any ideas what I'm doing wrong?  (By the way, this worked perfectly with Swift 3 &amp; Xcode 8, but now with Swift 4 and Xcode 9 the syntax has changed [Xcode walked me through the necessary code fixes/updates] - but the crashes keep happening.)</p>