Clean / define public programmatic interface

[junosuarez]

pseudocode:

// simple case
const Radar = require('radar')
const configuration = require('./config.json')

const server = new Radar(configuration)
server.ready.then(_ => console.log('radar started'))

// adding radar to an existing HttpServer instance
// this might have unexpected behavior due to middleware ordering, but is useful for things like testing
const Radar = require('radar')
const configuration = require('./config.json')
const http = require('http')

const server = http.createServer()
const radar = new Radar(server, configuration)

[junosuarez]

Main export should be RadarServer constructor