assetic.asset DI tag

I'm adding assetic tags to the [Symfony reference](http://symfony.com/doc/master/reference/dic_tags.html). For the ``assetic.asset`` tag, the only  use case I can think of is:

``` yaml
services:
    my_asset:
        class: Assetic\Asset\StringAsset
        arguments: [ 'alert("Hey dawg!");' ]
        tags:
            - { name: assetic.asset, alias: my_asset }
```

```twig
{% javascripts '@my_asset' %}
    <script src="{{ asset_url }}"></script>
{% endjavascripts %}
```

But this code produces an error in ``Symfony\Bundle\AsseticBundle\Routing::load()``. It happens because this method tries to get a formula for every asset, while our asset doesn't have a formula. This PR skips assets without formulae.