function Querystring(qs) {
  this.params = {};

  if (qs == null) {
    qs = location.search.substring(1, location.search.length);
  }
  if (!qs.length) return;

  // Turn <plus> back to <space>
  // See: http://www.w3.org/TR/REC-html40/interact/forms.html#h-17.13.4.1
  qs = qs.replace(/\+/g, ' ');

  // parse out name/value pairs separated via &
  var args = qs.split('&');

  // split out each name=value pair
  for (var i = 0; i < args.length; i++) {
    var pair = args[i].split('=');
    var name = decodeURIComponent(pair[0]);
    
    var value = (pair.length == 2)
      ? decodeURIComponent(pair[1])
      : name;
    
    this.params[name] = value;
  }
}

Querystring.prototype.get = function(key, defaultValue) {
  if (!this.contains(key)) {
    return defaultValue || '';
  }
  return this.params[key];
}

Querystring.prototype.contains = function(key) {
  var value = this.params[key];
  return (value != null) && (value != undefined);
}