There is a StringAsset in Assetic, but it is not usable. i don't know if there is any usage for the StringAsset. But for my usage it fails because of private variable.

```
$stringAsset = new \Assetic\Asset\StringAsset('need some content');
$stringAsset->setContent('test'); // fails -> Undefined property: Assetic\Asset\StringAsset::$content in [...]Assetic/Asset/BaseAsset.php on line 115
$stringAsset->content = 'test'; // fails -> Fatal error: Cannot access private property Assetic\Asset\StringAsset::$content
```
i'll catch this up in the ascetic project. until then, we need this separate Asset.