Developers banner and popup (bug 1052880)

https://bugzilla.mozilla.org/show_bug.cgi?id=1052880

FxA migration banners on devhub.


![fx-migration-desktop mov](https://cloud.githubusercontent.com/assets/211578/4412923/5044356c-44fe-11e4-8378-f7a5c9bdf695.gif)

I should probably copy over `storage.js` from commonplace but this was a quick refactor of the existing code already in the file.

I think this should stay in the module until we need it elsewhere at which point we can switch to `storage.js`.