Add JSFunctionWrapper in preparation for bindings-generator changes.
Merge branch 'functions' of https://github.com/colesbury/cocos2d-x into colesbury-functions
closed #2414: Bindings-generator could bind std::function argument now.
Merge pull request #3180 from dumganhar/colesbury-functions

closed 2414: Bindings-generator could bind std::function argument now.