Add `npm start`

Convenient for development, this command open the dev server & launch
jsbin server.
No it's definitly `&` that create the trick.
`./bin/jsbin` is a server that keep the terminal busy. 
Using `something &` start something in the background. Since we don't really want the jsbin server in the background (will be weird to have mixed output in the terminal), we launch the `open` in the background. But if we launch the background to early, the server is not ready yet (so `sleep 1` or `2` will do the job !).
I'll update the PR with a couple of parenthesis to make this more clear.