Bug 1049916 - Create Firefox Accounts smoke tests to validate user authe...

...ntication R=spenrose
I'm not sure what I think about having so many assertions in each test. I think it would be better to extract this out into per-screen utility functions--for instance, instead of `app.enterInput(selectors.emailInput, fxaUser.email(constants.USER_NEW))`, you could just say `app.enterNewEmail()`. This seems more consistent with how other apps' marionette tests work, eg video/marionette/lib/video.js.

Happy to chat in depth and/or help with organizing the code here :-)