Implement backend API to Angular.

### Demo: http://todomvc.stephenplusplus.com

What is this?! A todomvc PR from Stephen?! It's like 2013 all over again. And it feels great!

This PR contains part 1 and 2 on our mission to a TasteStacky TodoMVC: https://github.com/GoogleCloudPlatform/gcloud-node-todos/pull/11

The basic flow here is:

todomvc-common/base.js will ping `/api` to see if it gets a 200. If so, it sets a global "TODOMVC_API" to true. Any framework demo can then use that to save todos to the API if it exists, else fall back to the current system with localstorage. This PR has such an implementation for Angular.

Also tagging along in this PR is a simple `server.js` file and `package.json`. server.js is simply an express app that serves the static files from the repository. https://github.com/stephenplusplus/gcloud-node-todos/blob/master/server.js shows how gcloud-node-todos intends to use this. It hooks up its own api express app as middleware.
I really like this! Switching between the two backends like this is really cool. And it makes sure we have a proper encapsulation of the read/write logic.