add server to serve API docs

`npm start` will serve the API on `GET /`

A BE implementation can get the express instance with `require("todomvc-api").server`, and can get just the HTML to display the API spec with `require("todomvc-api").getSpecHtml`
I don't think the server needs to serve it via the browser. But if someone needs it, they can still `require()` it. We can add that in later if I'm overlooking something. Just wanted to merge to get the ball rolling :D