'Webroot' path and 'defaultImage' option

1. Change ```dirname(Yii::app()->basePath)``` to ```Yii::getpathOfAlias('webroot')```, there is a error when application folder is not located in webroot path
2. Add ```defaultImage``` option to extension, because It's not good to see ```PHP warning: getimagesize(): Filename cannot be empty``` in ```/drivers/Image.php``` file, when it's not exists.