Make it so -[ORSSerialPort dealloc] also closes the port if needed

[armadsen]

This is currently difficult because closing a port is an asynchronous operation, and therefore can't be triggered by the time the object is being deallocated. One possible fix is to create a helper object that is responsible for actually closing the port, whose lifetime is independent of the ORSSerialPort instance, and that can keep itself alive until the port is actually closed.