title: React Transition group not firing
body: <p>i tried to create a simple transition with react transition group, but i can't get it to work- transitions aren't working.
  i did used a unique key.</p>
  
  <p>for the example i just did a simple 2 image fade in fade out component:</p>
  
  <pre><code>var ReactCSSTransitionGroup = React.addons.TransitionGroup;
  
  var Image = React.createClass({
  
      getInitialState: function () { 
  
          return ({imglink: 'http://belaumi.com/wp-content/uploads/2014/11/3D-Animated-Frog-Image.jpg', status: 1})
      },
  
      update: function () {
  
          console.log(this);
          if (this.state.status==1)
          {
              this.setState({ imglink: 'http://www.codefuture.co.uk/projects/imagehost/demo/di/KXY1/Image-b%C3%A9b%C3%A9-facebook-8.jpg', status:2})
  
          } else {
  
              this.setState({ imglink: 'http://belaumi.com/wp-content/uploads/2014/11/3D-Animated-Frog-Image.jpg', status:1})
          }
      } , 
  
      render: function ()  {
  
          return (
        &lt;div&gt;
            &lt;div className='container'&gt;
            &lt;button onClick={this.update.bind(this)}&gt;Click&lt;/button&gt;
              &lt;ReactCSSTransitionGroup transitionName="example"&gt;
            &lt;img key={this.state.status} src={this.state.imglink}/&gt;
          &lt;/ReactCSSTransitionGroup&gt;
          &lt;/div&gt;
        &lt;/div&gt;
      );
    }
      });
  
      React.render(
          &lt;div&gt;
              &lt;Image/&gt;
          &lt;/div&gt;,
          document.getElementById('reactbody')
      )
  
  &lt;/script&gt;
  </code></pre>
  
  <p>i've also included proper css:</p>
  
  <pre><code>.example-enter {
      opacity: 0.01;
      transition: opacity .5s ease-in;
      -webkit-transition: opacity .5s ease-in;
  }
  .example-enter.example-enter-active {
      opacity: 1;
  }
  
  .example-leave {
      opacity: 1;
      transition: opacity .5s ease-in;
      -webkit-transition: opacity .5s ease-in;
  }
  .example-leave.example-leave-active {
      opacity: 0.01;
  }
  </code></pre>
  
  <p>any idea why this is not working? the image does switch, but no fade..</p>
  
  <p>thanks!</p>