JavaScript: Update example on CodePen.

[NodeGuy]

The JavaScript User Guide Getting Started page references a CodePen example that's currently broken. I made the following changes:

• Fixed it.
• Made it more idiomatic JavaScript.
• Made it graphical by rendering to the DOM instead of just printing on the console.

https://codepen.io/StubbornCreature/pen/dyedmKZ

Fixes https://github.com/dimforge/rapier.js/issues/48.

[ThierryBerger]

Thanks, we will avoid to link to a "foreign" codepen, but updating it is definitely needed ! I'll keep this PR open until resolved

[sebcrozet]

Thanks!