title: ios UITextField extension (subclass)
body: <p>I have a sign in form with inputs that have a white background and a red background. Instead of manually changing the backgroundfor each <code>UITextField</code> that has an error, can I extend the <code>UITextField</code> and add some extra functionality to it or is there a <em>better</em> way?</p>
  
  <p>Also, if I do a custom <code>UITextField</code>, is there a way do use that in <code>Interface Builder</code>?</p>