better support localStorage emulations (e.g. Node module 'dom-storage')

...by using the official getter/setter methods of the W3C DOM Storage API. This allows us to persist AlaSQL databases in Node applications as following:

```js
var DOMStorage = require("dom-storage")
global.localStorage = new DOMStorage("./app.json", { strict: false, ws: '' })

var alasql = require("alasql");
var db = new alasql.Database();

db.exec("CREATE localStorage DATABASE IF NOT EXISTS App");
db.exec("ATTACH localStorage DATABASE App");
db.exec("CREATE TABLE App.cities (city string, population number)");
db.exec("INSERT INTO App.cities VALUES ('Rome', 2863223), ('Paris', 2249975), ('Berlin', 3517424), ('Madrid', 3041579)");
```