Fiducial detection, first hack

I took my first stab at adding fiducial detection to touchlib today. I cam up with a simple algorithm that takes advantage of OpenCV's tree of contours (cvFindContours) which we are already using for blob detection right now. Basically it first detects whether a blob is square (which tells touchlib that this is a fiducial and not a finger press) then it adds up the detected shapes inside that square using a formula to get a unique identifier.