TranslatableParamConverter

**This PR can be merged only after https://github.com/fsi-open/doctrine-extensions/pull/45**

This PR introduces ``TranslatableParamConverter`` which allows converting URL parameters into action parameters of translatable classes. Let's assume we have a translatable entity class:

```php
# Acme/NewsBundle/Entity/News.php

namespace Acme/NewsBundle/Entity;

use Doctrine\ORM\Mapping as ORM;
use FSi\DoctrineExtensions\Translatable\Mapping\Annotation as Translatable;

/**
 * @ORM\Entity()
 */
class News
{

# ...

    /**
     * @Translatable\Translatable(mappedBy="translations")
     */
    private $slug;

    /**
     * @ORM\OneToMany(targetEntity="NewsTranslation", mappedBy="news", indexBy="locale")
     */
    private $translations;

# ...

}
```

some route:

```yml
# Acme/NewsBundle/Resources/config/routing.yml

acme_news_details:
    pattern:  /news/{slug}
    defaults: { _controller: acme.controller.news:detailsAction }

```

and finally some action in controller:

```php
# Acme/NewsBundle/Controller/NewsController.php

namespace Acme/NewsBundle/Controller;

use Acme/NewsBundle/News;

class NewsController
{
# ...
    public function detailsAction(News $news)
    {

    }
# ...
}
```

Without ``TranslatableParamConverer`` the default ``DoctrineParamConverter`` integrated in ``sensio/framework-extra-bundle`` will not be able to figure out how to find ``News`` object because the ``$slug`` member is not an ORM-mapped field. With ``TranslatableParamConverter`` everything will work as expected and you won't even notice the difference between regular ORM entity and entity with translatable properties.

Third type of checking if something is null ;)