(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```.
The ```$rowData``` is an Entity object, so we'll always need to build the getter for the field we want. Also, IMO, always having all DateTime properly formatted with a single setting seems good.

I thing we should keep the ```getValueForCell($field, $rowData)``` function (and make it private perhaps?) and call it by default. Getting something like this:
```php
protected function createExcelData(\PhpExcel_Worksheet $sheet)
{
    $row = 2;
    $results = $this->getResults();
    foreach($results as $rowData) {
        {%- set column = 0 -%}
        {%- for builder_column in builder.columns %}
            if (method_exists($this, 'format{{ builder_column.name|classify }}Value') {
                $data = $this->format{{ builder_column.name|classify }}Value($rowData);
            } else {
                $data = $this->getValueForCell('{{ builder_column.getter }}', $rowData);
            }

            $sheet->setCellValueByColumnAndRow({{ column }}, $row, $data);
            {% set column = column + 1 %}
        {%- endfor %}
        $row++;
    }
}
```
Still, I didn't grasp what would/should be the ```format**Value``` full method name for a field that's a property of an object association... guess I'll give this a try...