fix serious performance problem of Animate.

this is a serious bug, it cause massive cpu consume.
if we have 1000 animate in active scene, this pr can raise 5% performance globally, raise about 70% performance locally of Animate!!!!!!!!!!!! .
because it have massive calls cocos2d::Vector constructor and de-constructor. 
Vector's constructor will also call "addRefForAllObjects"(for loop)!!!!!


This problem is very, very serious,  and still has many similar errors in cocos2d-x.

we must use "auto&" sometimes, especially in update per frame. 
though we do not use exception, I suggest we use at, it is much more intuitive especially when the element does not exist with the key.

Many programmer don't know that operator[] will create a new element when it does not exist.

And operator[] let problem become much more hidden and harder to solve.

If we use map::at, it will cause app crash directly when element does not exist, and it is convenient for developer.

So In Cocos2d-x Engine, we should use map::at instead of map::operator[].


map::at
If k does not match the key of any element in the container, the function throws an out_of_range exception.

map::operator[]
If k does not match the key of any element in the container, the function inserts a new element with that key and returns a reference to its mapped value. Notice that this always increases the container size by one, even if no mapped value is assigned to the element (the element is constructed using its default constructor).