AsyncRx.NET Distinct and DistinctUntilChanged should be extension methods

#### Bug

The `AsyncObservable` class's static `Distinct` and `DistinctUntilChanged` methods are not extension methods. This means that given some:

```cs
IAsyncObservable xs = GetNums();
```

you can't write this:

```cs
xs.Distinct(); // or DistinctUntilChanged
```

You have to write this:

```cs
AsyncObservable.Distinct(xs);
```

This seems like a straightforward oversight. We just need to add the `this` keyword to these methods.