Enhancements for windows compilation

[SamyGr]

Hi, I have made some improvements to avoid some errors I encounter.

Explanations:

• CMakeLists.txt: add cmake minimum required necessary to compile
• CMakeLists.txt: add PRIVATE to all variables to avoid conflict with Stag's CMakeLists (I dont remember exact error).
• README.md: add some explanations (and mandatory import of opencv dll)
• test_warpper.py: add waitKey and destroyAllWindows functions to avoid python kernel crash.