Added a method resolving relative filesystem object path.

A use-case of the method: a web-accessible directory "httpdocs/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 = getPhotos();
    echo '<ul>'
    foreach ( $photos as $photo )
    {
        echo '<li><img src="' . $photo . '"></li>';
    }
    echo '</ul>';
}
````

The method does not yet support the case when the current working directory is higher than the file in the file system hierarchy and when the current working directory is outside of the file path but still may me useful in other cases.