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...
Update AUTHORS
Merge pull request #2500 from DarraghCoy/add_ccdirector_poptoscenestacklevel

Add CCDirector::popToSceneStackLevel(int level)
Update AUTHORS
Simplify CCPhysicsNode

No need to override nodeToParentTransform completely, only ensure
that position and rotation are up-to-date before performing normal
CCNode::nodeToParentTransform.

This change reduces the amount of cocos2dx and box2d specific code
significantly.
Merge branch 'simplify_physics_node' into nacl_changes2