update convenience constructors for CCSkeleton, CCSkeletonAnimation

We should use "self" instead of CCSkeleton.*, because the are lot situations, when we want to implement child of CCSkeleton.*. And we want to init childOfCCCkeleton.* using given convenience constructors.

I mean, when we creating childOfCCCkeleton, we want it to be childOfCCCkeleton, not CCCkeleton. 

P. S. sorry for my english.