Added a compilable asset collection.

This asset collection is useful for global minification and/or global compilation (tools like sweet.js), the filters are not transferred to child nodes.

This pull request is related to pull request #516

```php

$filter = new SweetJs\CompilerDumpFilter();

$asset = new CompilableAssetCollection();

// Adding sweet.js macros
$asset->add(new FileAsset('my/sweetjs/macros.js'));

// Adding enhanced javascript files
$asset->add(new FileAsset('my/enhanced/script.js'));

echo $asset->dump($filter);
```