Login option from everypage like in StackOverflow

Now my first and foremost thing I want to have done in my web application is to have something like in StackOverflow where I can login from any page. An Ajax or a jquery plugin for it to pop-up a modal box where I can fill in my details and bang!! I am logged-in.

Here's whats in my mind, login is one of the basic functionalities.. It is the one which enables session, which is the concept as important as breathing for any usable web application. And, me being a noob in, web application development, am banging my head to get this done. How the heck I do I get it done with Django and jquery is the biggest question. Biggest question of my life, probably now!