Using po4a to manage translations

This pull request adds a simple way to manage translations using the gettext system in the incarnation of Debian's po4a (http://po4a.alioth.debian.org/).

After a `sudo apt-get install po4a` one should be able to do `make translations` to generate `encryption_works.$LANG.md` files from the translations stored in the `po/*` files. Changes in the master file `encryption_works.md` are automatically incorporated into the `po/*` files and out-of-date translations are flagged as `fuzzy`.

There are still some problems with line breaks, that po4a adds in the process, but before spending more time with this, I first would like to get some feedback. Would you like to use this system to manage translations?

The idea to use gettext comes from the Tails projects homepage, which is managed using a very similar system in ikiwiki. The only negative side-effects is, that translators now need to use a tool such as `poedit` to do translations.