Implementing RelayCommand

This PR  implemented the RelayCommand class, which is widely used to represent unrouted command implementation of the ICommand interface and as you may know, the counterpart of it is the RoutedCommand class.

The command will be used to extract behaviors of views(eg. [Convert_Click](https://github.com/aliencube/Text-Encoding-Converter/blob/master/SourceCodes/02_Applications/TextEncodingConverter.WpfApp/MainWindow.xaml.cs#L92)) as @Jaewoo Ahn mentioned on [FB](https://www.facebook.com/groups/AspxKorea/permalink/746411725453059/?comment_id=746753728752192). I think that you can find many examples with just typing RelayCommand on search engines, but it is [a good example](https://github.com/Code52/MarkPadRT/blame/master/src/MarkPad/ViewModel/MainViewModel.cs#L41-L61) that implemented by @YoungjaeKim, which I just have found.