Modified me.loader.onProgress's arguments

me.loader.onProgress is now informed of which resource was loaded, just like me.loader.onError knows which one failed to be loaded.

I'm still not sure if there is any contribution convention (commit message, codestyle), so please feel free to criticize.

Also, I'm unsure wether I should also add this change in the broadcast event LOADER_PROGRESS.

This could potentially be useful for custom loading screens (which is what I have done with this change, as I am making a game that loads various areas named differently)