Tests are usually lauched from source directory, so additional unnecessary
files should be eliminated.

Alternative ways (command line):
- python -B ...
- PYTHONDONTWRITEBYTECODE=1 python ...

Usage example (bash):
$ OPENCV_PYTEST_FILTER=test_digits python test.py -v

Test launch script updated to run new tests as well.

At the moment tests requre samples/data copied to source location from master branch.

 - cv2.UMat implemented - python thin wrapper for UMat
 - no implicit copy from GPU to Host done, resulting UMat can be passed to next function without overhead
 - cv2.UMat.get() - to fetch data to Host
 - new tests covers: ORB, BFMatcher, goodFeaturesToTrack, calcOpticalFlowPyrLK

Add Python 3 support to the Python test.py script.

The print function is used in place of the print statement.

The urlopen function has been moved to urllib.request in Python 3, so
attempt to import it from either location.

TestCase.assert_() has been deprecated in place of
TestCase.assertTrue().

The tests all pass in both Python 2 and 3.

Also, removed the one from modules/python/src2/cv.py and cleared its
executable bit, since it's not a script.

Use github pat to get data for in python tests (files from code.opencv.org are received truncated sometimes)