Support using inherited interfaces in Hub<T>

#3311
Update Resources.resw so git doesn't annoy me

- The WinRT client has been superseded by the portable and store clients,
  so we should probably just remove this project.
Always subscribe to all ACKs on all servers

Prior to this fix, Groups.Add would fail to complete when there was no
subscription configured to receive the ACK on the server Groups.Add was
called on.
Prevent stack overflows when a Hub implements IHub

#3228
Make Hub<T>.Clients settable for testing

- Setting Hub<T>.Clients to null will really set it to its default value

#3299