function AverageObservable(source, fn) {
      this.source = source;
      this._fn = fn;
      __super__.call(this);
    }
Returns the maximum value in an observable sequence according to the specified comparer.
@example
var res = source.max();
var res = source.max(function (x, y) { return x.value - y.value; });
@param {Function} [comparer] Comparer used to compare elements.
@returns {Observable} An observable sequence containing a single element with the maximum element in the source sequence.