Focus the FxA popup email field (bug 1095056)

r? @cvan

`main.js` has an [event handler](https://github.com/mozilla/fireplace/blob/master/src/media/js/main.js#L132-L141) to hide the splash on the `'loaded'` event. Once it's hidden it fires `'splash_removed'` 1500ms later. Focusing on `'splash_removed'` gives a really weird behaviour. I'm using `setTimeout` to let the `main.js`'s `'loaded'` event handler run its course and hide the splash before focusing the email field.