Add an event to enable other modules to add data to the data layer

Hi Chris

Nice implementation. It would be cool to have an event for other modules to add additional data to the data layer. What do you think? 

An observer of this event could add data to the data layer as follows:
```php
    public function addDataLayer($observer) {
        $data_layer = $observer->getDataLayer();
        $data_layer->setData('MyAdditionalData', 'some data');
        return $this;
    }
```

Cheers,
Michael