Update Node.js sample for modern versions of Node and Express.

I updated the Node.js sample to work with the newer, stable versions of node and express.

I fixed the errors and deprecation warnings so that the sample will work out of the box for new users. There were errors for express (a multipart `bodyParser` is no longer bundled), and a few node deprecations as of 0.7 (`path.exists` is now `fs.exists`, and the `send` api changed).

I added a package.json for the dependencies so that users can simply run `npm install` to get the required dependencies (express & connect-multiparty). The `npm start` command can also be used to run the server, as an alternative to `node app.js`