Migrating Express 3 --> 4

#### What's this PR do?
This PR migrates our app from Express 3 to 4. [The changes from Express 3 to 4 are documented here.](http://expressjs.com/guide/migrating-4.html)

It does this in a number of ways: 

1. Express 4 has decoupled middleware formerly bundled within Express 3. We've required two new middleware packages, *errorhandler* and *body-parser*, within our `package.json` file. We've also edited our app-level config file in `app/config/index.js` to support usage of this middleware. 
2. It's recommended that we stop using the `response.send()` function to send response codes and messages like so: `response.send(200, 'OK'). Instead, Express 4 recommends that we send stand-alone statuses as `response.sendStatus(404)`, and codes with custom messages like: `response.status(406).send('Invalid mobile number.')`. We've found all references to response.send(code) or response.send(code, message) and changed them. 
3. Finally, we've changed the dummy response objects in our tests to include the `.sendStatus()` and `.status()` functions. 

#### Where should the reviewer start?
The area of code changed is pretty big here. First, check out the changed `package.json` file to see the middleware modules we've installed. Then, check out the files modified in regards to `response.send()`. Finally, check out how we've changed our test files. 

#### How should this be manually tested?
I've run an `npm test`. I've also successfully created and played through a BullyText game. 

#### What are the relevant tickets?
Closes #235.