allow full taxonomy term as keyword

This patch allows full (sanitized) term name for use as keywords. Our AdOps people don't want to have to traffic to truncated keywords (they would forget that the kw is "featur" instead of "features", for example). I also switched ereg_replace to preg_replace due to the former being deprecated in PHP 5.3 and supposed speed increases in the latter. I've replaced spaces with _ and allowed - in the full term names which doesn't necessarily line up with the truncate function, but it's what I do in my hook right now (to match the path and zone I'm setting). That "feature" could be removed without much trouble.