Add SkeletonAnimation and GLProgram to the list of extendable classes.

Hi.

In my case it was very useful to add ```SkeletonAnimation``` and ```GLProgram``` to the list of extendable classes...

For ```GLProgram``` extend I've added only ```cc.GLProgram.extend = cc.Class.extend```, but it was very hard to extend ```SkeletonAnimation``` class:

**This branch:**
1. I have added ```SkeletonAnimation``` to the list of extendable classes;

**Cocos2d-x branch:**
1. I've made empty constructor of ```SkeletonAnimation``` visible (public);
2. I've added ```initWithFiles``` method which will be called from:
3. I've added ```_ctor``` function.

What do you think about it?

Thank you!