Example from readme doesn't work (or needs elaborating)

The [example from the readme](https://github.com/etoccalino/django-rest-framework-httpsignature#example-usage--session-wcurl) calls `Base64` and `Hmac` as command line utilities or shell functions. Could you add information on how to get these utilities?
It also passes the literal string `SSS` in the second command. I guess this should be `$SSS`?