Consider adding a logout endpoint/helper

[toolness]

When originally creating this package, a logout endpoint seemed redundant because it had nothing to do with UAA.

However, I've since learned that there is actually a UAA endpoint to log-out a user, which invalidates their UAA token, among other things. See 18F/tock#814 for an example of this.

So it might be nice for cg-django-uaa to provide either a logout endpoint or a helper of some sort to assist with this.