Updated Tutorial to use werkzeug.security module

[mandarvaze]

Existing code does not work anymore.

1. string.letters not available in python3
2. string functions in encrypt requires encode("utf-8")
3. Finally, salt used only during setting the password and not when checking, so passwords never match

Using well-tested library functions is preferred.

[dkellner]

Thanks for your PR! I've merged after rebase to current master and fixing some minor whitespace issues.