This PR was squashed before being merged into the 2.0.x-dev branch (closes #1309).

Discussion
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add a namedForm() method in FormTrait

I do prefer using the namedForm builder.

Just add the `$this['form.factory']->createNamedBuilder(...)` in `Silex\Application\FormTrait`:

```php
    /**
     * Creates and returns a named form builder instance.
     *
     * @param string                   $name
     * @param mixed                    $data    The initial data for the form
     * @param array                    $options Options for the form
     * @param string|FormTypeInterface $type    Type of the form
     *
     * @return \Symfony\Component\Form\FormBuilder
     */
    public function namedForm($name, $data = null, array $options = array(), $type = null)
    {
        if (null === $type) {
            // BC with Symfony < 2.8
            $type = class_exists('Symfony\Component\Form\Extension\Core\Type\RangeType') ? 'Symfony\Component\Form\Extension\Core\Type\FormType' : 'form';
        }

        return $this['form.factory']->createNamedBuilder($name, $type, $data, $options);
    }
```

Commits
-------

c8e61fc2 add a namedForm() method in FormTrait