added 'ellipse' method to canvasRenderingContext2D

[FayCarsons]

fix #1552

[hhugo]

Thanks, I've added two commits to make the CI happy, I'll merge after it runs.

(I've noticed that ellipse was used in the examples/hyperbolic/hypertree.ml example and implemented as

let ellipse_arc c cx cy rx ry start fin clock_wise =
  c##save;
  c##translate (Js.float cx) (Js.float cy);
  c##scale (Js.float rx) (Js.float ry);
  c##arc
    (Js.float 0.)
    (Js.float 0.)
    (Js.float 1.)
    (Js.float start)
    (Js.float fin)
    clock_wise;
  c##restore

One could cleanup this example now that we have ellipse binding.