Removed deprecated app window singleton attribute

The `singleton` attribute is deprecated since Chrome 34. Let's clean our repository!
Source: https://developer.chrome.com/apps/app_window#property-CreateWindowOptions-singleton

TBR=@mangini

![image](https://cloud.githubusercontent.com/assets/634478/4122695/34024b12-32c7-11e4-8b87-e45002339be4.png)