(v1.0) Re-organized ExcelBuilderAction.php.twig

Added  a ```getValueForCell``` protected function so that extending the values format for the cells will be easier.

For instance if we need to format the value of a field which we know it's a currency, we can easily extends this method and add those special cases. E.g.:
```php
protected function getValueForCell($field, $rowData)
{
    switch($field) {
        case 'price':
        case 'productionCost':
            $getter = 'get'.ucfirst($field);
            $data = number_format($rowData->$getter(), 2, ',', '.');
            break;
        default:
            $data = parent::getValueForCell($field, $rowData);
            break;
    }
    return $data;
}
```

I don't know if this is the best example, but this can also be used to render boolean field with some text other than ```TRUE/FALSE```.
@Restless-ET Ok I found it. Its defined [here](https://github.com/symfony2admingenerator/AdmingeneratorGeneratorBundle/blob/c5471f8bd5d291becf7d9f7fc30cf0472bd5cb67/Builder/BaseBuilder.php#L27)