Proj01

[xingxinggeng]

1. fixed issue Encapsulate variables and use getters & setters #42 , which encapsulated variables by using getter and setter methods.
2. fixed issue Adjust the window where the user can grab the ball #72 , which adjusted the window for user to grab balls in a more reasonable range.
3. fixed issue Change Y Ball velocity when ball hits paddle #92 , which changed the velocity of the ball slightly when the ball hits the paddle.
4. added JUnit tests Add Junit tests #13 for Ball, Paddle, Pong classes.