Added a method resolving relative filesystem object path.

A use-case of the method: a web-accessible directory "photos" contains a number of photos which should be shown on the web-page. A script scans the directory and retrieves relative paths of photos. Then photos may be referenced by their relative paths.

Example:
````php
public function getPhotos()
{
    $photos = array();
    $photosDirectory = Yii::app()->file->set("photos");
    $photosPaths = $photosDirectory->getContents(false,'jpg');
    foreach ( $photosPaths as $photoPath )
    {
        $photoFile = Yii::app()->file->set($photoPath);
        $photoFileRelativePath = $photoFile->getRelativePath();
        $photos[] = $photoFileRelativePath;
    }
    return $photos;
}

public function printPhotos($photos)
{
    echo '<ul>'
    foreach ( $photos as $photo )
    {
        echo '<li><img src="' . $photo . '"></li>';
    }
    echo '</ul>';
}
````