Issue 351: PHP 5.3.3 compatibility issue with filter constant

Fix #351 

@fjarrett I just removed it. It's not the most useful one: http://pl1.php.net/manual/en/filter.filters.sanitize.php

I'm not sure if that's reasonable to copy `filter_var` all the way. I'm actually not into those constants thing at all, we could provide an alternative with much better API.