function SampleObservable(source, sampler) {
      this.source = source;
      this._sampler = sampler;
      __super__.call(this);
    }
Records the timestamp for each value in an observable sequence.

@example
 1 - res = source.timestamp(); // produces { value: x, timestamp: ts }
 2 - res = source.timestamp(Rx.Scheduler.default);

@param {Scheduler} [scheduler]  Scheduler used to compute timestamps. If not specified, the default scheduler is used.
@returns {Observable} An observable sequence with timestamp information on values.