Fix for bug with disappearing colon digit and adding simple demo

When clearing a digit the width parameter of clearRect function has to be just digit width. Before you were clearing area of size digit width * digit position (so for example when second digit was cleared it was deleting 2 digits, itself and the next one, when third digit was cleared it was deleting itself and 2 other digits (digit width * 3))

I have also added a simple demo directly to repo, I think you should add all your demos inside.