make 'closed' event available to Client

[villadora]

Some times I want to make some cleanup after the client is closed but due to current code, Client.close() method is only push request to queue but there is no way for Client to know when the ConnectionManager just have done the work.

So I think 'closed' event is also interested by Client and should not ignored in code.

I have my own fork t to just do simple work to just emit 'closed' when the close job is done.

Please see in villadora/node-zookeeper-client@ae37fc7

[alexguan]

Please see my comments in PR #15