pay from the web! no mozApps (bug 1109947)

@muffinresearch r?

This lets us test payments directly in the browser using the hosted example app. It still doesn't open a window yet, that will come next.
You could use `window.location.origin when available. 

When it's not (Gecko < 21) you can use protocol + `window.location.host` which will add the port when present. e.g: 

```javascript
window.location.protocol + '//' + window.location.host;
```
See http://jsbin.com/falufuluba/1/edit?js,console