Added async auto subscription

I was a bit heavy handed with refactoring the AutoSubscriber to avoid repeating code, also i have introduced a breaking change by adding a method to the IAutoSubscriberMessageDispatcher. Another questionable implementation detail is that i could have dealt with the auto async subscriptions from the existing AutoSubscriber.Subscribe method, instead i added anther method, AutoSubscriber.SubscribeAsync.

If you are happy with the changes, i can update the wiki if you like.