tfa_u2f

A quick PoC of integration between the tfa module and U2F.

No guarantee that it actually works as it was thrown together quickly (see below).

Main things that needs fixing
- Figuring out how and whether to include the u2flib_server (see https://github.com/Yubico/php-u2flib-server)
- Same for https://demo.yubico.com/js/u2f-api.js
- Start storing the users information as entities or in a custom schema instead of variables
- Make sure it all actually works :)