Refactor out WebsSocketManager from WebSockerServer

[zemeolotu]

This PR refactors out and exports a new standalone WebSocketManager class from WebSocketServer.

This refactor allows perspective to be remotely hosted in node servers other than WebSocketServer. WebsocketManager will manage all websocket connections and messages from a perspective websocket client.

As an example, here's how you'd use it in an express server

const app = require('express')();
const expressWs = require('express-ws')(app);

const manager = new WebSocketManager()
manager.host_table(name, table)

app.ws('/perspective/subscribe', ws =>  manager.add_connection(ws));

This PR also adds a node.js express server example hosting a perspective table

[texodus]

Looks good! Thanks for the PR!

Appreciate the examples cleanup, in particular.