title: Read string from clipboard
body: <p>How can I read a string from the clipboard and write it on a button's title?  I can copy a string to the clipboard with:</p>
  
  <pre><code>UIPasteboard.generalPasteboard().string = "Hello world"
  </code></pre>
  
  <p>But how do I read that string from the clipboard and assign it to a String?</p>