gpio: adding interrupt

this is inspired by Dave Cheney's gpio library and his work on EPOLL
gpio: added missing function in fakeDigitalPin
embd, rpi: determine pi revision, make rev2 pinmap default

added a function to determine the revision of the pi the program is
running on from "Revision" field in /proc/cpuinfo, also made the rev2
gpio pinmap the default
Merge pull request #13 from alsm/master

embd, rpi: determine pi revision, make rev2 pinmap default
Merge pull request #11 from SjB/gpio/interrupt

gpio: adding interrupt
doc: update CONTRIBUTORS