namespaced alias class

presuming a sane pre-existing autoloader for namespaced modules, this is immediately deployable with a nice namespaced alias class, eg:

```php
$purifier = new \HTMLPurifier\Processor();
$html = '<strong>a string<br></strong>'; // (notice the dirty <br> that should be <br />)
echo $purifier->purify($html); // outputs: <strong>a string<br /></strong>
```