osx icon badge support - setBadgeLabel

osx is supported only, I'm not sure is there way to implement on other platforms
```js
var gui = require('nw.gui'); 
var win = gui.Window.get();
win.setBadgeLabel("3");
```
![image](https://f.cloud.github.com/assets/880901/2471880/1af7d3fc-b02d-11e3-841a-daa28609fd4c.png)