Minimize adherence to express

Hello,

I originally forked to remove some warnings when using express >=4.0.0 concerning the use of `send()`

The point of this refactor was to minimize adherence to express-specific API and use mostly built-in `http` stuff.

I ended up refactoring some tests in order to be sure not to break anything. Also running npm install --save-dev ended up reformatting the whole `package.json` file