Added viewmodels and views from the Exam Preparation demo.
See last commit
Merge branch 'master' of https://github.com/BlueForeverI/XamlChat.Client

Conflicts:
	Xaml.Chat.Client/ViewModels/ViewModelBase.cs
	Xaml.Chat.Client/Xaml.Chat.Client.csproj
	Xaml.Chat.Client/Xaml.Chat.Client.v11.suo
	Xaml.Chat.Client/obj/Debug/DesignTimeResolveAssemblyReferencesInput.cache
	Xaml.Chat.Client/obj/Debug/Xaml.Chat.Client.csproj.FileListAbsolute.txt
	Xaml.Chat.Client/obj/Debug/Xaml.Chat.Client_MarkupCompile.i.cache