Initialize the WebSocket server in the Server constructor

[lpinca]

The kind of change this PR does introduce

• a bug fix
• a new feature
• an update to the documentation
• a code change that improves performance
• other

Current behaviour

Described in #473.

New behaviour

Makes a plain Engine.IO server, not bound to any HTTP server, work again.

Other information (e.g. related issues)

Fixes #473.
It should also fix cases where an already listening server is used which is currently equally broken.

'use strict';

const eioc = require('engine.io-client');
const eio = require('engine.io');
const http = require('http');

const server = http.createServer();

server.listen(3000, () => {
  const engine = eio.attach(server);

  engine.on('connection', (socket) => {
    socket.on('message', (data) => socket.send(data));
    socket.on('close', () => console.log('close'));
  });

  const socket = eioc('http://localhost:3000');

  socket.on('open', () => socket.send('foo'));
  socket.on('message', (data) => console.log(data));
  socket.on('close', () => console.log('close'));
});

[darrachequesne]

@lpinca thanks a lot!