work on obj_detect.py

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)

samples/python2/common.py

@@ -2,6 +2,8 @@ import numpy as np
 import cv2
 import os
 
+image_extensions = ['.bmp', '.jpg', '.jpeg', '.png', '.tif', '.tiff', '.pbm', '.pgm', '.ppm']
+
 def splitfn(fn):
     path, fn = os.path.split(fn)
     name, ext = os.path.splitext(fn)