Python samples adapted for Python3 compatibility

Common fixes:
- print function
- int / float division
- map, zip iterators in py3 but lists in py2

Known bugs with opencv 3.0.0
- digits.py, called via digits_video.py: https://github.com/Itseez/opencv/issues/4969
- gaussian_mix.py: https://github.com/Itseez/opencv/pull/4232
- video_v4l2.py: https://github.com/Itseez/opencv/pull/5474

Not working:
- letter_recog.py due to changed ml_StatModel.train() signature

samples/python2/plane_ar.py

@@ -22,6 +22,9 @@ Select a textured planar object to track by drawing a box with a mouse.
 Use 'focal' slider to adjust to camera focal length for proper video augmentation.
 '''
 
+# Python 2/3 compatibility
+from __future__ import print_function
+
 import numpy as np
 import cv2
 import video
@@ -97,7 +100,7 @@ class App:
 
 
 if __name__ == '__main__':
-    print __doc__
+    print(__doc__)
 
     import sys
     try: