Zend\Filter\File\RenameUpload - Added possibility to maintain original file extension

When using the Zend\Filter\File\RenameUpload filter on a file upload (that can have any extension) the extension is lost if the target does not specify it.

Added a new option (use_upload_extension) that, when specified, maintains the original extension.

Example :

If the user uploads a .png image **test.png** :
```php
<?php
$filter = new RenameUpload(array(
    'target'  => pictures,
    'use_upload_extension'  => true,
    'randomize'  => true,
 ));
```
The filter will return something like **pictures_ad321e2r5ty5s.png**

Works with randomize on and off.