Manage multiple payment accounts

https://bugzilla.mozilla.org/show_bug.cgi?id=990441
https://bugzilla.mozilla.org/show_bug.cgi?id=989946

Not quite the 3-in-1 I had hoped, Boku raises NotImplemented.

![screenshot 2014-04-09 13 39 19](https://cloud.githubusercontent.com/assets/211578/2659668/02977656-c017-11e3-9ab1-53fd9ad59550.png)