Added support to Context2D 'transform' method

I need to use the context `transform` method in one of my projects.
I noticed that such function was not provided by Game Closure, but since the other transformations (`transform`, `scale`, `rotate`) were already present, this one was trivial to implement.
Here is the patch for `native-core`, the one for `native-android` is ready as well, so I could successfully test this patch.