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.
Fix #338
Regenerated delivery scripts

Commits:
 * Fix #338
Ref #106 - Updated DO defaults for advertiser emails
Merge branch 'master' of github.com:revive-adserver/revive-adserver
Ref #106 - Fixing tests
Ref #106 - Fixing test failure
Ref #106 - Last try to fix the failing test
Fix #352 - Delivery regex issue on pgsql
Removed useless phpSniff demo file
Regenerated plugins

Commits:
 * Fix #352 - Delivery regex issue on pgsql
 * Removed useless phpSniff demo file
Notes for #352
Fixed #356 - IDN support for ck.php destinations
Regenerated delivery scripts

Commits:
 * Fixed #356 - IDN support for ck.php destinations
Fix settings w/underscores in the Options UI
Added CORS header
Regenerated delivery scripts

Commits:
 * Added CORS header
Clean up changes for the release notes.
Added a note to the copyright file so that we don't have to remember to ask people about being added to the copyright file!