#143 add support for new dot-based calibration target

[Nathan Oostendorp]

OpenCV 2.3 adds support for a new, more accurate and robust dot-based calibration grid. We should add support for it in our calibration routine.

Major advantages include:
- can be combined with a reversed grid (white dots on black background) to do calibration in half the frames
- is much faster find function
- dot centroid has higher subpixel accuracy than the spCorners algorithm