title: Knockout-2.2.0, subscribe get value before change AND new value
body: <p>jsfiddle link: <a href="http://jsfiddle.net/T8ee7/">http://jsfiddle.net/T8ee7/</a></p>
  
  <p>When I call Knockout's subscribe method is there a way I can get both the previous and new value? Right now, I can only call get these values separately.
  I want to trigger some code if the old and new value are different.</p>
  
  <p>I suppose I could do the following, but it can get messy...
  (<a href="http://jsfiddle.net/MV3fN/">http://jsfiddle.net/MV3fN/</a>)</p>
  
  <pre><code>var sv = sv || {};
  sv.PagedRequest = function (pageNumber, pageSize) {
      this.pageNumber = ko.observable(pageNumber || 1);
      this.numberOfPages = ko.observable(1);
      this.pageSize = ko.observable(pageSize || sv.DefaultPageSize);
  };
  
  
  var _pagedRequest = new sv.PagedRequest();
  var oldValue;
  _pagedRequest.pageNumber.subscribe(function (previousValue) {
      console.log("old: " + previousValue);
      oldValue = previousValue;
  }, _pagedRequest, "beforeChange");
  
  
  _pagedRequest.pageNumber.subscribe(function (newValue) {
  
      console.log("new: " + newValue);
      if (oldValue != newValue) {
          console.log("value changed!");
      }
  
  });
  
  _pagedRequest.pageNumber(10);
  _pagedRequest.pageNumber(20);
  ​
  </code></pre>