title: Paypal Express Checkout with ActiveMerchant as in Shopify
body: <p>I'm successfully using Paypal Express Checkout in a marketplace web app passing for every seller the three parameters to the PaypalExpressGateway (login, password and signature). 
  So, I see that Shopify lets the seller set only the e-mail address of  the Paypal account requesting to grant scott_api1.jadedpixel.com as the API Partner Username. 
  I could't find anything in the ActiveMerchant doc on how I can do it...Someone can help? </p>
  
  <p>Thanks a lot,
  zetarun</p>