Allow more control over the animation loop

[BinaryMuse]

doDrawLoop currently decides how often to redraw planets (based on d3.timer). As per part of #12, it would be nice to allow end-users to customize the draw ticks. We should extract the main draw body:

planet.context.clearRect(0, 0, canvas.width, canvas.height);
for (var i = 0; i < hooks.onDraw.length; i++) {
  hooks.onDraw[i]();
}

and allow end-users to call it whenever they want.