Added minimal webcam monocular example

[danifm01]

In order to offer a faster trying of the algorithms, I add an example that works with (virtually) any USB webcam. It relies on VideoCapture class of OpenCV.

[IamShubhamGupto]

@danifm01 do you think you can gui support for Mac with a similar example? it works because I have disabled the Viewer thread otherwise it complains that the GUI has to be on the main thread.

Thanks