Add login / logout / get profile example

The 'quick start' code snippets provided in the documentation can be quite confusing, so I created a fully functional simple script for basic authentication (login), deauthentication (logout) and profile retrieval (GET /me). It helped me understand how the SDK works and I think it might help others too.

Usage and setup are explained in the script's comments, but it is as simple as cloning the repo (and installing its dependencies with composer) on a public web server folder (whose domain must be allowed on the app settings) and setting the 4 parameters: app id, app secret, login url, logout url. Then you can check out the SDK functionality just by hitting the script's URL on your browser.