Implement Investigation Update Notifications

[eljobe]

My team would find it useful to be notified as changes to TeamCity investigations occur.

I'm happy to implement the missing methods on your SlackNotificator.java class if you're open to taking contributions.

By the way, I really like your Slack integration much better than the others I've seen. The configurability is much-more sensible. Really good idea building it as a Notificator instead of some other integration point.

[aclark09]

Appreciate it, and yes we're always open to pull requests, especially since we haven't had as much time as we'd like to make upgrades. I think investigation notifications would be a welcome addition.