Allow configuration of the square size in stereo_calib.

https://github.com/Itseez/opencv/issues/6456

Previously, there's no way to the user see the found corners, i've changed that.

In a cout, are write that: "average reprojection err = "
But it isn't a "reprojection error" at all, it is a mean of each EPIPOLAR error, wich occur when the product x' * F * x is not equal to zero.
(x and x' are the same points in the right and left scene)
(the RMS that explain the average absolute reprojection error is given by the return of the stereoCalibrate() function)

At least, i think it's interesting to initialize the camera matrices before.

Thank you all for this amazing code. Apologize my weak english.

Also fixed some typos and code alignment
Also adapted tutorial CPP samples
Fixed some identation problems

added cvSave(contours) to the contours demo; added rational model demonstration to stereo_calib; fixed formatting in imgproc/types_c.h

make cv::calibrateCamera, cv::stereoCalibrate and their C counterparts return the standard RMS error.