Elastic bounceback for ScrollView

Add iOS-like elastic bounceback.
If you scroll content of scroll view out of insets,
its content will be scrolled 0.35 of user scrolled distance.
Merge commit 'refs/pull/5856/head' of git://github.com/cocos2d/cocos2d-x into merge5856
closed #4448: Elastic bounce back effect support for cocos2d::extension::ScrollView.
issue #4448: Respond to move event  if dragging is enabled, no matter whether mouse is in the content of cocos2d::extension::ScrollView.
Merge pull request #5869 from dumganhar/iss4448-scrollview

closed #4448: Elastic bounce back effect support for cocos2d::extension::ScrollView.