Authentication

In conjunction with 1749aa32c25d8eee9ca0753cb55bd7d134f320b7, these commits add authentication functionality to the project.

1. User calls `stack-authenticate`
2. User approves the application
3. User is redirected to our `gh-pages` branch (thanks, @Bruce-Connor!)
4. User is instructed on that website and within Emacs to paste the displayed code into the `read-string` that is waiting for it
5. Access code is cached in "auth.el"

Unfortunately, this can't really be tested with CI since it requires authenticated user interaction.