[WIP] Introducing GridType's

@dylanschoenmakers , what do you think about this? It pretty much follows the way Symfony's FormType's work. 

This way you could create GridType's like:

```php
namespace AppBundle\Datagrid\GridType;

class ContentGridType implements GridTypeInterface
{
    /**
     * {@inheritDoc}
     */
    public function buildGrid(DatagridBuilderInterface $builder)
    {
        $builder
            ->addColumn(........)
            ->addColumn(........)
        ;
    }
}
```