Wilbur,
A logout function could very easily be implemented. Login, I assume, is a function of the values of various cookies that you set. If you want someone to logout, just remove those cookies. I believe you can use your same function, but modify it to remove instead of place cookies.
Keep up the good work. It is one of the best website I have ever seen. I think NI should patent the interface and sell it (and send me a portion of the proceeds for suggesting it...)