B+ Revision check & Pins

This was my attempt at creating a PR for issue #8. I was not sure what to do with the hackablePins array as I'm not entirely sure what the purpose of that array is. Hope this helps get things up to date for the B+. Changes seem to be working okay with mine when it comes to reading pins, have no started messing with blinking pins.
The Raspberry Pi Revision 2.0 introduced a new header to the PCB. [This header is called P5 and consists of 8 pins](http://www.raspberrypi-spy.co.uk/2012/09/raspberry-pi-p5-header/). The new added GPIO pins are [28, 29, 30 & 31](http://elinux.org/RPi_Low-level_peripherals#GPIO_hardware_hacking). Since you use a Revision 2.0 Raspberry Pi, can you confirm this, and then maybe add this pins to the REV2 list please?