Composer ScriptHandler to run assetic:dump from a script hook

Introduces following configuration options:

```php
array(
    'assetic-dump-asset-root' => null,
    'assetic-dump-force' => false
);
```

The following ScriptHandler can be added:

```json
{
    "post-install-cmd": [
        "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::dumpAssetic"
    ]
}
```

This is a copy of #34, which apparently was abandommed by its original author. The code was believed to be lost due to bad rebase, but I found [this gist](https://gist.github.com/2725096).