Small patch

Moved fillstyle assignment in shape.draw so that a shape's draw function can include its own extra fillstyle assignments. E.g.:

    ABM.shapes.add("filledRing", false, function(c) {
        ABM.shapes.circ(c,0,0,1);
        c.fillStyle = "rgb(255,255,255)";
        c.fill();
        c.beginPath();
        ABM.shapes.circ(c,0,0,.8);
    });

Also added a randomHSBColor function to util (maybe should be made more customizable?).