Added Unity TDD Tests and cleaned up pfio.h to be a minimal interface

Hi Thomas, 
I've just started playing with the piface and was looking at your supplied C wrapper functions. 
I've had a go at cleaning them up. Initially fairly simple (i.e. you should never use char for 8-bit data as it's neither signed or unsigned) and used the useful C header <stdint.h> for uint8_t as a better type for byte data.
I removed all the stuff from the header that shouldn't really be there (you only put stuff in a header of use to the client).
I haven't changed any basic behaviour but have some plans.
If you've never see Unity as a C based TDD framework it's vey useful.
Thanks for doing the difficult bit,
Regards, Niall.