Cocos2d-x: changes for JSB support.

Hi.

These changes are needed for supporting **extend** of this classes in [Cocos2d-x Javascript Binding](https://github.com/cocos2d/cocos2d-js):

1. All constructors and init methods must be in ```CC_CONSTRUCTOR_ACCESS``` scope instead of ```protected``` because of **JSB** binding generator`s needs;
2. I'm add init functions for all constructors of ```SkeletonRenderer``` because we need to init the class after calling empty constructor (That is how **JSB** works).

We can't modify sources of **spine-runtimes** according to the license conditions so in  [Cocos2d-x Javascript Binding](https://github.com/cocos2d/cocos2d-js) we very need these changes.

Hope you understand.
Thanks.