Fixed Asset vars in url generation

I needed to create a new `Twig_Node_Expression_Raw` class to use `Compiler::raw()` in `compile`:
```php
class Twig_Node_Expression_Raw extends Twig_Node_Expression
{
    public function __construct($value, $lineno)
    {
        parent::__construct(array(), array('value' => $value), $lineno);
    }

    public function compile(Twig_Compiler $compiler)
    {
        $compiler->raw($this->getAttribute('value'));
    }
}
```

Now this piece of code work:
```html
{% javascripts
"%kernel.root_dir%/../vendor/jms/twig-js/twig.js"
output='js/twig.js'
%}
<script language="javascript" type="text/javascript" src="{{ asset_url }}"></script>
{% endjavascripts %}

{% javascripts
"%kernel.root_dir%/../template/hello_world.twig"

output='js/template.{locale}.js'
filter="twig_js" vars=["locale"]
%}
<script language="javascript" type="text/javascript" src="{{ asset_url }}"></script>
{% endjavascripts %}

<script language="javascript" type="text/javascript">
    alert(Twig.render(hello_world));
</script>
```

This PR should fix the following issues:
- https://github.com/symfony/AsseticBundle/issues/136
- https://github.com/symfony/AsseticBundle/issues/182
- https://github.com/schmittjoh/JMSTwigJsBundle/issues/21