fixed #1748: Separating js extension from cocos2dx.hpp/cpp.

* Generating `jsb_cocos2dx_extension_auto.hpp/.cpp`. It was separated from original `cocos2dx.hpp/cpp`.  Developer could remove redundant jsbinding native codes now.
`cocos2dx.hpp/cpp` ~> `jsb_cocos2dx_auto.hpp/cpp`
* Adding `jsb_cocos2dx_extension_manual.hpp/cpp`