[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;

use Opifer\CrudBundle\Datagrid\GridType\GridTypeInterface;

class ContentGridType implements GridTypeInterface
{
    /**
     * {@inheritDoc}
     */
    public function buildGrid(DatagridBuilderInterface $builder)
    {
        $builder
            ->addColumn(........)
            ->addColumn(........)
        ;
    }
}
```

And in the controller the datagrid will be called like;

```php
$datagrid = $this->get('opifer.crud.datagrid_factory')->create(new ContentGridType(), $entity);
```