Update assetmanager.js

Currently adding assets from subfolders of the <i>path</i> setting is not supported. Applications often structure script files in subfolders to allow a better overview (community also asked for this: https://github.com/mape/connect-assetmanager/issues/47). 

The pull request introduces a new parameter <i>recursive</i>. If this parameter is set to true, files from path are loaded recursively. All other mechanism are applied as before. An example usage could look like this: 

```javascript
var assetManagerGroups = {
    'js': {
        'debug': false,
        'route': /\/static\/js\/app.bundle.js/
        , 'path': './public/app/'
        ,recursive: true
        , 'dataType': 'javascript'
        , 'files': [/[a-zA-Z/]*.js/], 'postManipulate': {
            '^': [
                assetHandler.yuiJsOptimize
            ]
        }
    }
};
```

The parameter is necessary because loading files from sub directories could potentially break existing usages.