Merge remote-tracking branch 'origin/master'
Add level function
When user gets the star, the level upgrades. At the same time, the
speed of enemy is increasing.
Add Star class and add direction feature to Enemy
implement Player class and its instance
implement movement of player and collision method
Implement Enemy class
generate 7 Enemy instances
Change the size of canvas
Change the layout of canvas and add some characters.
Init game engin