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
fixed #2099:fixed a bug in ScriptingCore::evalString not setting the outval
Merge pull request #2503 from minggo/evalString

fixed #2099:fixed a bug in ScriptingCore::evalString not setting the out...
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Merge pull request #2500 from DarraghCoy/add_ccdirector_poptoscenestacklevel

Add CCDirector::popToSceneStackLevel(int level)
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Merge remote-tracking branch 'upstream/master'