Fixes #14 Add Unit Tests for App Class and GamePanel class, refactor GamePanel class

app/build.gradle

app/src/main/java/brickbreaker/Ball.java

app/src/main/java/brickbreaker/DefaultGameRenderer.java

app/src/main/java/brickbreaker/GamePanel.java

app/src/main/java/brickbreaker/Interfaces/GameRenderer.java

app/src/test/java/brickbreaker/AppTest.java

app/src/test/java/brickbreaker/GamePanelTest.java