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>