Give sentinel objects (or at least NEED_DATA and PAUSED) unique types

[njsmith]

@dabeaz has requested this, to make it easier to take the thing that comes out of next_event() and dispatch it using functools.singledispatch or something like:

 class Handler(object):
     def handle_events(self):
            while True:
                   evt = conn.next_event()
                   name = 'handle_' + type(evt).__name__
                   getattr(self, name)(evt)

     def handle_Data(self, evt):
           ...

     def handle_EndOfMessage(self, evt):
          ...

     def handle_Response(self, evt):
          ...

     def handle_NEED_DATA(self, evt):
          ...

Seems reasonable enough to me.

[njsmith]

Fixed by #18