Little performance improvements in SpriteBatchNode

_descendants: Array -> std::vector<Sprite*> since retain/release is no needed
Code is cleaner
Removes unused method (bug inherited from cocos2d-iphone)
Adds more doxygen strings
removes 'private' section from SpriteBatchNode
compiles Linux / Windows
Merge pull request #3530 from ricardoquesada/sprite_batch_node_perf_improvements

Little performance improvements in SpriteBatchNode