Revised ViewFactory 

- Added support for multiple pages for the same ViewModel and also adjusted the ViewModelNavigation to support that matter. The first page is the default one so if a view isn't specified when navigating to a ViewModel the first one is always selected by the viewfactory createpage method.

- Added support for adding a NavigationPage around the called page e.g. :
Navigation.PushAsync<SomeViewModel>(null, true); // will add navigation page
This will fix issue / bug: #194 Enable cache is thus fully functional now.

- Added support to add a page + viewmodel as single instance. So using Resolver.Resolve will get you always the same instance. 

Needs a lot of testing. I have tested it with the sample project with Android and iOS and it seems to work :)