Use ReactiveList in Conductor.Collection

Shouldn't the conductors' list of items be a reactive list if we are merging the best of both worlds?

__UPDATE:__
This would be a breaking change as it changes the API of the public Items list.

```c#
public IReactiveList<T> Items
{
    get { return items; }
}
```

We could create a subclass of `ReactiveList` that implements `Caliburn.Micro.IObservableCollection`.