[PM] - [18n] Support for i18next translations + German translation

This is a follow-up of #39.
[See the page live](https://googledrive.com/host/0B986Uw1nLGY7XzNlY2hXVXI5X1U/index.html?lang=de) (make sure you enable insecure content).

Changes made:
* [i18next](http://i18next.com) added. It does all the translation stuff for us, and is pretty fast.
* German translation added (`locales/de/translation.json`)
@pthiess Could you please review the translation?
@jadbox Could you please take a look at the structure of the translation object keys, the changes done to the `.html` files and so on?

Other changes:
* Minor changes to the english translation
* Use jQuery. Yes, using http://i18next.com without jQuery wouldn't even make sense. (We can refactor some code later to actually use jQuery in other places)
* Remove `jquery.reveal.js` - it wasn't needed and caused nothing but errors.