Check for OpenCV 2, 3 and 4

[kopetri]

Since there are multiple versions of opencv available, you should change your version tests cv2.__version__.split(".")[0] == '3' to something like int(cv2.__version__.split(".")[0]) >= 3

[mpatacchiola]

Hi @kopetri you are right, however at the moment we have not tested if everything is fine when using OpenCV > 3, therefore we cannot guarantee the correct functioning of the library.

If you want to contribute you are more than welcome to check if everything runs smoothly and send a pull request with the updated flag in all files. This should not take too long.