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/digits_video.py

@@ -1,5 +1,8 @@
 #!/usr/bin/env python
 
+# Python 2/3 compatibility
+from __future__ import print_function
+
 import numpy as np
 import cv2
 
@@ -22,7 +25,7 @@ def main():
 
     classifier_fn = 'digits_svm.dat'
     if not os.path.exists(classifier_fn):
-        print '"%s" not found, run digits.py first' % classifier_fn
+        print('"%s" not found, run digits.py first' % classifier_fn)
         return
     model = SVM()
     model.load(classifier_fn)