Removed what appears to be dead/unreachable code within constants.php and 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
Merge pull request #334 from zytzagoo/fixing-constants

Removed dead/unreachable code in constants.php

Also added a new IS_WINDOWS constant that might come in handy.