Enable config'd postMessage in paymentSuccess/Failure (bug 1101004)

r? @kumar303 
well, huh, I'm not sure if specifying a target as ``window.opener.location`` is allowed. I have been getting the fxpay side working using a dummy page on people and I get this error in Chrome:

Uncaught SecurityError: Blocked a frame with origin "http://people.mozilla.org" from accessing a frame with origin "http://localhost:3000". Protocols, domains, and ports must match.

I guess we have to use target ``*`` and have fxpay do the filtering? I changed my dummy code to that and it worked. Maybe there is another way.

My first popup patch landed if you want to experiment with it: https://github.com/mozilla/fxpay/pull/48