Profile expiration and bot detection enhancements

* Change preg_match to str search for performance and readability as recomended by note in http://us1.php.net/preg_match
* Check if user agent string is defined and not empty prior to to text comparisons against it
* Hook to allow override of bot detection will always be available instead of only when wpec decides visit is not a bot
* advance the profile expiration time whenever the last active time is being set to the current time
* timestamps can be passed as timestamp strings or as unix times