Add class_alias() function as a built-in

Hi, I've finally added class_alias() function as a built-in, which I proposed on facebook-page for HipHop-PHP and below issue.
https://github.com/facebook/hiphop-php/issues/705

The purpose of adding this feature is that many useful php frameworks are able to work on hiphop-php.
(ex.  FuelPHP)

I'll be very happy if you review and merge this pull request when you have time.