Handle simulated payments (bug 1045905)

@muffinresearch r?

Pretty much same screen as before (which should really look more like a real purchase page):

![simulate](https://cloud.githubusercontent.com/assets/55398/3851093/40697f42-1e90-11e4-8822-a6b154c0e2ee.png)

The Webpay API for this will be added in [bug 1050593](https://bugzilla.mozilla.org/show_bug.cgi?id=1050593)