Added master detail support

[krdmllr]

Added support for the master detail page type.

• MvvmNanoMasterDetailPage is an abstract implementation providing multiple methods to hook up a custom layout
• MvvmNanoDefaultMasterDetailPage builds on the abstract implementation and uses a listview to display the available details - it should be good enough for most use cases
• Details are added to the MvvmNanoMasterDetailPage with the AddDetailData<TViewModel>(MvvmNanoMasterDetailData data) method provided by the page itself
• You can navigate between the details using NavigateToAsync<ViewModelOfOtherDetail>()

Additional:

• SetUpMainPage in MvvmNanoApplication supports all page types, MvvmNanoContentPage/TabbedPage/MasterDetailPage now
• Completle recreated the demo to use the latest Xamarin.Forms template
• Added an example for the MvvmNanoMaterDetailPage
• Added an example for the MvvmNanoTabbedPage

[krdmllr]

AppVeyor is failing because the PR includes C#7 features which AppVeyor can't handle with the current Environment.

[krdmllr]

Ping @bbqchickenrobot, you were interested in the last PR.