Reinstate WPSEO_URL constant.

Although WordPress SEO itself doesn't use this constant, a number of Yoast's premium add-on plugins do (WPSEO Local for example). Without this constant in place, stylesheets are not loaded. The result is broken looking option screens for the add-on plugins. I've attached a screenshot of the WPSEO Local options screen below to demonstrate the effects...

![wpseo-local](https://f.cloud.github.com/assets/148775/1582657/531c59fe-51f2-11e3-8817-7f2922c9936f.png)