Single Log Out support

[ralequi]

Added requested feature on #58 .

This commit changes and adds many things that have been required to implement the single sign out:

• Added some defaulted settings to simplify configuration (see simple_sso/settings.py):
• Added APP simple_sso.sso_client to send client logouts to server.
• Added Middleware simple_sso.sso_client.middleware.PostAuthenticationMiddleware to have a "keep-alive" behaviour and checks every X seconds if the current user is still connected on the server. (Defaulted to 60 but can be 0 to check on every client request)
• Fixed tests for Django 3.x

I think the usage is clear on the modified README. Please, tell me if it's not clear enough or you see any code-issue.

Everything was tested locally and with a couple of remote servers. Testing environment:

• python 3.8 (ubuntu20) & python 3.9 (Fedora)
• Django 3.2.3 (ubuntu20) & 3.2.5 (Fedora)