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.
How about I boost this up to the conn level, where there's already a counter for in-flight messages?

Also, what do you think about tacking on a time-out where one gives up on any outstanding messages and just sends the CLS?