Refactor and slim down app 

This PR is heavy, but mostly because it removes some `node_modules` dependencies, which is a lot of files. It might be easier to just [go visit the branch](https://github.com/18F/analytics-proxy-nodejs/tree/style) and browse around.

This brings the app in line with my take on Node style, and tries to take advantage of Node's flexible functions in order to flatten and simplify the app. Happy to sit and go over the various changes, if they're not self-explanatory.

One major thing is that I removed all the modules and code related to rendering HTML and managing cookies and favicons and stuff. Since this app is, at least to start with, an all-API app, it can ditch a lot of that weight.

I removed some other stuff, like the `debug` library, because it seemed like it might have been from boilerplate and not a deliberate preference. If I was wrong, obviously let's reinstate it.

Fixes #3.