Added const qualification to some CCNode's getters

As we want to be able to copy construct CCNode's subclasses we want to be able to do this:

    class SubNode : public cocos2d::CCNode
    {
    public:
        SubNode(const SubNode& other)
        {
            this->setContentSize(other.getContentSize());
        }
    };

So i've added constness to the following getters:
<ul>
<li>getTag()</li>
<li>getContentSize()</li>
<li>getChildrenCount()</li>
</ul>
Of course this are not the only methods that need const added, but seem the most obvious ones.
We cannot add const to all of them because implementations of them would need to be const as well.