Add option for changing windows icon

This PR add an option `winIco` for changing the windows executable icon. This is working, I just test it using:

```
var NwBuilder = require('./');
var nw = new NwBuilder({
    files: '/Users/samypesse/Desktop/Projects/GitBook/editor/**/**', // use the glob format
    platforms: ['win'],
    winIco: '/Users/samypesse/Desktop/Projects/GitBook/editor/src/resources/images/icons/512.ico'
});

// Log stuff you you want
nw.on('log',  console.log);

// Build retruns a promise
nw.build().then(function () {
   console.log('all done!');
}).catch(function (error) {
    console.error(error);
});
```

I also fix a typo: "platform" instead of "plattform".

I have a PR pending on https://github.com/atom/node-rcedit/pull/1 that adds the support of mac and linux to node-rcedit.