Issue #22: Add a score

Whenever the ball bounces on a pad, increase the score by 10 points.
If is on a wall, I decrease the score by 2 points.
This value was defined in the config by the new object named 'Game.Config.Score'

The default color of the score is red, defined in the style.css file.

To conclude, I created a new object score in the main to manage the previous and current score.