Update AUTHORS
Adding popSceneBack() function to CCDirector

This function deletes the running scene and returns the actual scene
back to the caller. This way, we can simply use it to perform a
transition animation, like this:

    CCDirector::sharedDirector()->replaceScene(
            CCTransitionSlideInL::create(0.5,
            CCDirector::sharedDirector()->popSceneBack()));

Signed-off-by: Rodrigo Goncalves de Oliveira <rodrigo.goncalves@openbossa.org>