Add EWG.org to USERS.txt

Blurb received earlier from Justin Miller:

  Environmental Working Group <http://www.ewg.org/>

    The Environmental Working Group (EWG) uses the power of public
    information to protect public health and the environment. Boost is used
    to cache all public-facing pages on the site (13,000+ and counting) and
    has been critical in sustaining EWG's large amounts of traffic since the
    site relaunched using Drupal in early 2007. EWG frequently receives
    traffic from multiple press outlets on a given day and Boost allows EWG
    to manage its infrastructure in-house and at a fraction of the price
    that would otherwise be required.