Support for GPIOs

If you build the kernel with CONFIG_GPIOLIB enabled, you can control them.
This is working 100% correct for me, but I can't test i²c, so you have to integrate (and test) it yourself.
Also, I don't know whether it works on older nspire models (non-CX), please test it before you pull.

BTW: The intention on github isn't right at all, with my editors it's displayed correctly.