Added support for Revision 2 boards

Used the gpio mapping described by [drogon](https://projects.drogon.net/raspberry-pi/wiringpi/pins/) and [raspberry spy](http://www.raspberrypi-spy.co.uk/2012/06/simple-guide-to-the-rpi-gpio-header-and-pins/) - fixes issue #1 on my Revision 2 board.