Cls issue 70

Attempting to handle issue #70 by building on @jonesetc close patch in issue #100 to fold in a CLS message back to the nsqd.
* Adds a cls message
* Adds send_cls method to async conn
* Adds close method to the Client class, now you can close Publishers too
* Subsequently tweaks the Reader class

Close stuff happens in a lot of places, probably missed something.

I need to figure out how to roll in some tests as well.
Is this outstanding message accounting an issue for just Writers? Readers? Or both?

Also, is an outstanding count good enough, or do individual messages need to be tracked?