Removed what appears to be dead/unreachable code within constants.php an...

...d added a new IS_WINDOWS constant.

Reasoning behind the removal:
* DIRECTORY_SEPARATOR is always defined (at least according to the PHP manual)
* PATH_SEPARATOR exists since php 4.3.0, and Revive requires PHP 5.1.4+ at the very least so...

Source: http://www.php.net/manual/en/dir.constants.php