Update documentation to reflect the second parameter of passed in cha…

…nge events.

api.html

@@ -71,7 +71,8 @@ <h4 class='fn-signature'>
         <a name='IvyAttr.prototype.set' href='#IvyAttr.prototype.set'>IvyAttr.prototype.set(value)</a>
       </h4>
 
-      <p>Sets the internal value of the attribute.</p>
+      <p>Sets the internal value of the attribute, and emits a <code>change</code>
+event with the new value and the old value as the arguments.</p>
     </div>
         <div class='fn'>
       <h4 class='fn-signature'>
@@ -89,8 +90,10 @@ <h4 class='fn-signature'>
 The most common event is <code>change</code>.
 </p>
 <pre><code>var a = Ivy.attr(4);
-a.on(&#39;change&#39;, function(value){ console.log(&#39;New value is&#39;, value); });
-a.set(5); //=&gt; &#39;New value is 5&#39;</code></pre>
+a.on(&#39;change&#39;, function(value,oldValue){ 
+  console.log(&#39;New value is&#39;, value, &#39;old value was&#39;, oldValue); 
+});
+a.set(5); //=&gt; &#39;New value is 5 old value was 4&#39;</code></pre>
     </div>
         <div class='fn'>
       <h4 class='fn-signature'>

ivy.js

@@ -29,11 +29,12 @@ function IvyAttr(value, parseFn){
 };
 
 /**
- * Sets the internal value of the attribute.
+ * Sets the internal value of the attribute, and emits a `change`
+ * event with the new value and the old value as the arguments.
  */
 IvyAttr.prototype.set = function(value){
   var oldValue = this.value;
-  
+
   value = this.parseFn ? this.parseFn(value) : value;
 
   if (oldValue === value) return this;
@@ -56,8 +57,11 @@ IvyAttr.prototype.get = function(){
  * The most common event is `change`.
  * 
  *     var a = Ivy.attr(4);
- *     a.on('change', function(value){ console.log('New value is', value); });
- *     a.set(5); //=> 'New value is 5'
+ *     a.on('change', function(value,oldValue){ 
+ *       console.log('New value is', value, 'old value was', oldValue); 
+ *     });
+ *
+ *     a.set(5); //=> 'New value is 5 old value was 4'
  * 
  */
 IvyAttr.prototype.on = function(event, fn){