Added ViewModelViewHost for Winforms
* Added RoutedViewHost for Winforms + tests
* Added RxApp.DefaultExceptionhandler to ViewModelViewHost
Cosmetics
Merge remote-tracking branch 'origin/pr/396'
Allow unsubscribing from RegisterAsync

Note that this still PermaRefs for methods that don't return a value,
since I suspect many people are not correctly subscribing to these
methods.