added the feature of listen public events

I have added the feature of listen public events (at this moment only the update event) because I needed it in my app.

In my situation I need to make some UI changes in others components whenever the date range is changed.

The callback to event is passed by the options in the moment of creation:

```javascript
$('input').daterangepicker({
  events: {
    update: function (startDate, endDate) {}
  }
});
```

I hope that it be usefull.