Add emit on user change to Slack client

[lambtron]

Hi @paulhammond and @grantmd,

Good news! We've been able to successfully migrate our bot, Hermes, from Hipchat to Slack without much difficulty!

One thing we'd like to add to your Slack client is to add an emitter to fire on new user or user change. This way, when Hermes is running, a new user who joins the organization would be able to interact with Hermes (Hermes would dynamically add the new user to the list of all users).

We were thinking of just adding a single line as below:

Here:
https://github.com/slackhq/node-slack-client/blob/master/src/client.coffee#L386

Thoughts?

/cc @stevenmiller888

[grantmd]

Yep, great idea. Added here: 4bf44ee

Thanks!