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?
this makes me nervous because what if the payment window encounters an error and never sends a postMessage? One way to address that is to use a global like ``fxpay.pay._pollIntervalRef``. That way we'd avoid multiple overlapping intervals.