This doesn't match the Rx.Net signature: http://msdn.microsoft.com/en-us/library/hh211932(v=vs.103).aspx

```C#
public static IObservable<IGroupedObservable<TKey, TSource>> GroupByUntil<TSource, TKey, TDuration>(
	this IObservable<TSource> source,
	Func<TSource, TKey> keySelector,
	Func<IGroupedObservable<TKey, TSource>, IObservable<TDuration>> durationSelector
)
```

Why is `maxGroups` being added to the signature?