Add a button on the progress view.

Hi, 

I want to cancel uploading/downloading URL session.
So, I added the button to the progress view.

thanks.

![ios simulator screen shot 2014 09 06 15 51 50](https://cloud.githubusercontent.com/assets/4963478/4174477/7dbe3f18-3592-11e4-9b38-a40527d575bb.png)