Fix back button for scene 2 > scene 1. bug 824356

Add code so back button will take user to scene 1 from scene 2.
Adds '#download' hash to URL for modern browsers (IE9+).
Begin dl if arriving with #download in URL. Bug 824356

Two minor concerns:

- Possible race condition with pulling correct download link from
  DOM prior to window.load. Could duplicate DOM query in window.load
  to be safe.
- Install images may not be fully loaded if on slower connection. Not
  sure we can do much here.
Bug 824356: refactor new.js
Merge branch 'mozilla-firefox-new-b-back-button'