Added Beaglebone code to sensors.c & actuators.c

So I've probably broken something here with my half-remembered CITS1002
code, but hopefully it works. I've added real analog/digital sensor
tests to sensors.c and real digital/pwm actuators to actuators.c. These
reference gpio.c and pwm.c for functions. I still need to add documentation
but it should be fairly self-explanatory.