Tracking mouse events in Artist

[alexandremeunier]

Following #159, we now have a list of all shapes currently drawn on canvas. We need to be able to pass mouse events callbacks as arguments when drawing the shapes, in a way similar to that:

this.artist.rect(10, 10, 10, 20, {
  fillStyle: 'red',
  click: function(event) { console.log('click'); }
});

Definining mouse event callback will cause the artist to automatically track the shape, and store a reference to the callbacks.

Then, we need to capture the click, mouseenter, mouseleave events on the $canvas object (in the Artist), and check whether the mouse was in one of the shapes we are tracking. For all the shapes in which the mouse click/hovering happened, we need to trigger the callback defined before.