Add CCDirector::popToSceneStackLevel(int level)

This replicates a change made to cocos2d-iphone in the following commit:
http://github.com/cocos2d/cocos2d-iphone/commit/59c5272bffc2c9fb4685ff5665af33d0595dabf1

The cocos2d-iphone change was originally discussed in the following pull request:
http://github.com/cocos2d/cocos2d-x/pull/2210