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

In an observer it would be something like this:
```php
    public function addDataLayer($observer) {
        $data_layer = $observer->getDataLayer();
        $data_layer->setData('MyAdditionalData', 'some data');
        return $this;
    }
    }
```
Merge pull request #3 from ufirstgroup/gtm-dispatch-event

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