Opencv3

[ocean1992]

Hi, ankush,

I'm trying to use your project to generate some ocr training images and i found your code only support opencv2

Since now opencv3 is the popular version (pip does not provide opencv2), so i transform cv2 code to cv3.

A requirement file and readme is updated

[ankush-me]

@ocean1992: thank you for the pull request, and my apologies for the long delay in responding! I will review it in a few days and get back to you.