Small refactoring and Cancellable commands added

[kirbex]

• Made a small refactoring.
• Classes AsyncCommand & CustomAsyncCommand no more sealed.
• Create new base class AsyncCommandBaseExtended
• Added two classes: CancellableCustomAsyncCommand & CancellableAsyncCommand. They are usefull because I don't need to create every time I need to cancel command separate property in my viewmodel with CancelCommand type. One can use it in view like SomeAsyncCommand.CancelCommand or in code behind or viewmodel like SomeAsyncCommand.Cancel().