Readme clarity

I had some difficulty implementing this in Swift and implementing my own resized background image, so I added an example to the readme.

Also, in my testing on iOS 8B2, I found that this package only adds transparency and doesn't blur the image (as below). We should let iOS 8 become stable before worrying about this issue.

![photo jun 26 8 48 47 pm](https://cloud.githubusercontent.com/assets/1320475/3407023/41ff8cba-fd9e-11e3-871a-f674360b9f30.png)