[Routing] Add support for anchor in url generation

I an propose add `_anchor` or `_fragment` parameter to `generate` method, like in Magento http://www.magentocommerce.com/wiki/5_-_modules_and_development/reference/geturl_function_parameters

``` php
<?php

$router->generateUrl('topic.view', array('topic_id' => 1, '_anchor' => 'comment-2')); // '/topics/1#comment-2'
```
