added square_size cmd line param for calibrate.py
added _coverage.py script to compute cv2 api coverage by samples (just for interest, may be removed later)

fixed warnings in gbt; added insertChannel() and extractChannel(); made the code "rand{u|n}(arr, <number>, <number>)" work properly.

removed trailing backspaces, reduced number of warnings (under MSVC2010 x64) for size_t to int conversion, added handling of samples launch without parameters (should not have abnormal termination if there was no paramaters supplied)