Use splinter + headless selenium to test the sketch renderization

[berinhard]

After #103, the pytop5js.py file now has more logic attached to it. Since this file is transcribed to JS code, the proper way to test it would be with acceptance tests. For those we can use the splinter project with a headless selenium webdriver.

The main tests scenarios would be:

• Create a simple sketch to draw a circle in the middle of the canvas and make sure the circle is there by checking the DOM;
• Make sure we can use Python's map function and p5.js one;
• Make sure we can use Python's filter function and p5.js one;
• Make sure we can use Python's set function and p5.js one;