Make ProofOfExistence translatable

Uses the strategy described here for translations on AppEngine + Jinja: http://mikeshilkov.wordpress.com/2012/07/26/enable-jinja2-and-i18n-translations-on-google-appengine/

Translating.md describes how to update and add new translations: https://github.com/mapmeld/proofofexistence/blob/translatable/translating.md

* Adds babel and gaepytz libraries (this is why it is such a big PR)
* Adds a few sample translations (Spanish) in locale/es/LC_MESSAGES/messages.po
* Add a translation.js for messages in JavaScript
* Set language based on user selection, URL, cookie, or language sent to server (in that order)

You can try it out on http://map-poe.appspot.com?lang=es - I do not know enough to translate much, but you can see it on the homepage and about.