provide node path on configuration

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs? | No
| Applies to    | all
| Fixed tickets | 

If you no provide node path in assetic configuration, UglifyCssFilter throw a  `RuntimeException: Path to node executable could not be resolved.` 
I would write something like

> Assetic tries to find the ``node`` binary automatically. If it cannot be found, you'll be able to configure its location using the ``node`` key: