add itsdangerous library, add lib to PYTHONPATH

With ./lib/ being a proper python module included in $PYTHONPATH,
YouCompleteMe (Jedi to be exact) can now pick up code suggestion for
those libraries in vim.
use hmac-based authentication

User access tokens are now not stored on server. They're now HMAC-signed
tokens stored on client side. Basically inspired by
http://lucumr.pocoo.org/2013/11/17/my-favorite-database/