boilerplate code for bouncing balls
Define Ball model

* It knows:
	- its coordinates
	- its color
	- its radius
	- how to draw itself
Define Ball fillStyle using its color; close draw path
Draw Balls when canvas is clicked

* Create BouncingBalls collection
* Define BouncingBalls#add that creates a new Ball and adds it to the Collection
* Define BouncingBalls#draw to draw all balls into its canvas
Separate interval function to be #tick, it will be responsible for moving balls AND drawing
Draw Ball as soon as click event happens (no need to wait till next tick)
Define random color when Ball is created
Add border to canvas (its wrapper) - remove papayawhip color, too bright