title: Knockout custom binding update not firing
body: <p>With the following code, I would expect my update function to execute every time the viewModel.item observable is updated. I can see my init and update functions firing on page load as expected, but not when my button is clicked which updates the observable's value.</p>
  
  <p><strong>Markup:</strong></p>
  
  <pre><code>&lt;button id='addButton'&gt;item++&lt;/button&gt;
  &lt;br/&gt;
  &lt;span&gt;viewModel.item = &lt;/span&gt;
  &lt;span data-bind='text: $data.item(), bind: viewModel.item'&gt;&lt;/span&gt;
  </code></pre>
  
  <p><strong>Script:</strong></p>
  
  <pre><code>$(document).ready(function() {
      $('#addButton').click(function() {
          viewModel.item(viewModel.item() + 1);
      });    
      var viewModel = {
          item: ko.observable(1)
      };        
      ko.bindingHandlers.bind = {
          init: function(element, valueAccessor) {
              alert('init');
          },
          update: function(element, valueAccessor) {
              alert('update');
          }  
      };        
      ko.applyBindings(viewModel);
  }); 
  </code></pre>
  
  <p>I've browsed some of the similar questions and haven't found a comparable example. I've created a JSFiddle <a href="http://jsfiddle.net/vMD74/12/" rel="noreferrer">here</a>.</p>