How to use this i2c NES controller?

I have this controller which has a Wiimote expansion plug and uses I2C just like the Wii Nunchuk does. At least the PCB is hinting at that:

Now I would like to use this controller with one of my Arduinos. Is there maybe already a library for this? Any ideas?

I figured it out. I'll try to turn my code into a library.