Adding Inference Script to produce video sample

[vinesmsuic]

• Added a script for inference.

usage: example_video.py [-h] --vid VID [--output OUTPUT] [--dont_show]
GazeTracking Video Inference.
optional arguments:
  -h, --help       show this help message and exit
  --vid VID        path to video
  --output OUTPUT  inference video name. Only support .avi extension due to
                   OpenCV 3.4
  --dont_show      hide imshow window

• Also did a minor change in example.py to prevent distorted screen when the webcam is not available

ret, frame = webcam.read()

# Quit the program if no webcam is found
if ret == False:
    break

• Added documentation