Add Subscription type to the API

[wallyqs]

The subscription methods were modeled after the original Ruby client:

  # Simple publisher and async subscriber via coroutine.
  sid = await nc.subscribe("foo", cb=message_handler)

  # Stop receiving after 2 messages.
  await nc.auto_unsubscribe(sid, 2)

but similar as with other clients, we should change the API so that it is more consistent with the Go client:

  sub = await nc.subscribe("foo", cb=message_handler)
  await sub.auto_unsubscribe(2)
  await sub.unsubscribe()

[wallyqs]

Added to v1-dev branch here: #175