Complete web purchase from message (bug 1101995)

@muffinresearch r?

I've only tested with a dummy html page but it should work with your patch!
I find the indentation here hard to parse at a glance. I wonder if it could be more readable like so:

```Javascript
it('rejects unknown statuses', function(done) {
  fxpay.pay.acceptPayMessage(
    makeEvent({status: 'cheezborger'}),
    defaultOrigin, 
    function(err) {
       assert.equal(err, 'UNKNOWN_MESSAGE_STATUS');   
       done();
    });
});
```