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.
The patch to `native-core` (to add `context_2d_transform`) is ready as well, here is the one for `native-android` that adds the function to the context through JNI.