Fix getModuleDir call

> Warning: Missing argument 2 for Mage::getModuleDir(), called in /app/code/community/Zendesk/Zendesk/controllers/Adminhtml/ZendeskController.php on line 18 and defined in /app/Mage.php on line 338

This is found when accessing anything that calls the `JWT.php` helper.

I have found this in CE 1.7.0.2. The issue was introduced in 74757fe.

Uses `DS` as directory separator for compatibility reasons.

Reference: http://docs.magentocommerce.com/Mage_Core/Mage_Core_Model_Config.html#getModuleDir

Replaces #10 with a single change PR.