Use shared NodeLoaderLibrary in CCBReader bindings

In my project, I have native C++ node with JS bindings and CocosBuilder plugin. Currently, there is no way to add custom node loader to CCBReader instance, used in JS bindings - default loader library is always created. This patch replaces creation of new loader library with shared instance, so we can now call:

```cpp
CCNodeLoaderLibrary::sharedCCNodeLoaderLibrary()->registerCCNodeLoader("MyNode", MyLoader::loader());
```

on app startup and MyLoader and custom node it loads will be available in JS.