User provided initial state for ReactiveAsyncCommands

ReactiveAsyncCommands when driven by an IObservable defaults initially
to reporting CanExecute as true before any events pass through the
canExecute observable.  This change keeps that default behavior, but
allows the user to change this default to false if appropriate.
User provided initial state for ReactiveCommands

ReactiveCommands when driven by an IObservable defaults initially
to reporting CanExecute as true before any events pass through the
canExecute observable.  This change keeps that default behavior, but
allows the user to change this default to false if appropriate.
Merge pull request #150 from sillyotter/fix-reactive-async-command

User provided initial state for Reactive[Async]Commands