Javascript helper window.t(key) for i18n in js (pre-req for [#71270474])

Idea stollen from :
http://stackoverflow.com/questions/2701749/rails-internationalization-of-javascript-strings
https://gist.github.com/oleander/6a3f1b3a4cf8de889e34

https://www.pivotaltracker.com/story/show/71270474
Completes: [#71270474] "Submit your prediction" to "Submit"

https://www.pivotaltracker.com/story/show/71270474