Add ellipse

[GMartigny]

Ellipses are one of the most common shape and it's easy to draw in canvas.

However, I don't know how it should integrate in the current dependency tree.

Current tree:

• Component
  • Arc
    • Circle

Possible solutions:

1. Split arcs and full ellipses

• Component
  • EllipseArc
    • CircleArc
  • Ellipse
    • Circle

2. Add another branch

• Component
  • EllipseArc
    • Ellipse
  • Arc
    • Circle

(1) IMO, arcs (ellipse or circle) should be treated by default as stroke (like lines and paths). Which mean that they could even inherit from Line.

(2) In the other hand, circle is just a specific arc (just as square is a specific rectangle) as much as a specific ellipse. Since double inheritance is impossible in JS, it's not an option.

[GMartigny]

Fixed by 57c6b47.