Translate landing page using Localize.js

This commit integrates Localize.js (https://localizejs.com) on whispersystems.org and adds English / Spanish language navigation links to the footer.

I added some extra logic to automatically detect the user's browser and translate to their preferred language.

If you haven't heard of Localize.js, it's a javascript library for translating websites on the client side. I set you up an account for you on https://localizejs.com/ where you can add more languages and edit / order translations. 

If you're interested in merging this, ping me at brandon@localizejs.com and I'll send you the login info for your account :)