Add closure detection WiP

Despite having created a much smaller test I'm still seeing this error on the tests:

![1__bash](https://cloud.githubusercontent.com/assets/1514/5494713/fbaccec8-86ee-11e4-8203-77256aeac320.png)

Running the test on its own passes.

@kumar303 any thoughts on what might be causing it?
that would also be a little nicer because you wouldn't have to expose ``processWebPayment`` and call it directly. You could do ``fxpay.purchase()`` and assert that the window gets closed after calling ``finishPurchaseOk()``. I think you could manually change ``fakeWindow.closed`` in the ``payCompleter`` callback from that example, like:
````
helper.finishPurchaseOk('<receipt>', {
  productData: {webpayJWT: fakeJwt},
  payCompleter: function() {
    fakeWindow.closed = true;
    simulatePostMessage({status: 'ok'});
  }
});
````