Add Window and Buffer overloads to allow specifying maximum time length and element count when window/buffer opening is controlled by another sequence

Basically, add the following overloads to **Window** (respectively for **Buffer**):

``` csharp
public static IObservable> Window(
this IObservable source,
IObservable windowOpenings,
TimeSpan timeSpan,
IScheduler scheduler
)

public static IObservable> Window(
this IObservable source,
IObservable windowOpenings,
int count
)

public static IObservable> Window(
this IObservable source,
IObservable windowOpenings,
TimeSpan timeSpan,
int count,
IScheduler scheduler
)
```

This would allow using external events to specify the opening of overlapping windows while using the traditional element count and time length conditions to automatically close the windows.

Note that so far as I understand it, there is no trivial implementation for the **_count**_ overload, even if you wanted to implement this using the existing overload with window closing factory. It would be exceedingly useful to have this built into the framework for completeness.