Bad URL generatetion on WINNT

Hi,

the generated URL for css and js is wrong, bug is in minifyx.class.php on ln. 57

```php
$cacheFolderUrl = DIRECTORY_SEPARATOR . str_replace(MODX_BASE_PATH, '', $this->config['cacheFolder']);
```

On Win this variable has this value:
$this->config['cacheFolder'] = E:/htdocs/modx-revolution/www_root//assets/components/themebootstrap/min\

So $cacheFolderUrl will have this value:
\/assets/components/themebootstrap/min\

You can't use DIRECTORY_SEPARATOR constant for URLs, you should use char __/__ or in MODx is better way, you can use const MODX_BASE_URL