$.fn.observe = function(interval, callback) {
  return this.each(function() {
    var vm = (this.tagName == 'FORM') ? 'serialize' : 'getValue', lastValue = $(this)[vm](), self = this;
    setInterval(function() {
      var value = $(self)[vm]();
      if (value != lastValue) {
        callback(self, value);
        lastValue = value;
      }
    }, interval * 1000);
  });
};