Customize Application Icon

``` JavaScript
dialog({
  icon: 'icon.png', // relative to __dirname by default
  title: 'A Title',
  msg: 'some message',
  timeout: 10,
  checkboxes: [
    'first box',
    'second box'
  ]
}, function (error, button, checkboxes) {
    // do stuff with the result.
})
```
