title: Omnipay with PayPal express checkout
body: <p>I'd like to integrate PayPal Express Checkout with Omnipay, but documentation is totally uncompleted. How can I run methods like 'setExpressCheckout', 'doExpressCheckout' and 'getExpressCheckout'? Also, how to redirect all PayPal calls to IPN listener? Does Omnipay provide all these methods or just part of them? Has anyone had chance to use this library with Express Checkout?</p>
  
  <p>Regards!</p>