Improved alpha animation of popUpView

Previous alpha animation made visible shadows, buttons and other possible subviews used to compound the popUpView: 

![iOS Simulator Screen shot Apr 3 2013 10 22 55 PM](https://f.cloud.github.com/assets/612990/336032/b0ce12c4-9c9c-11e2-95b4-5723915a4860.png)

By rasterizing popUpView, all its subviews fade as a single object, resulting in a smoother alpha animation.

![iOS Simulator Screen shot Apr 3 2013 10 23 37 PM](https://f.cloud.github.com/assets/612990/336031/a8607960-9c9c-11e2-8981-3cea24f218a3.png)