Use Library for Posting to Slack

[eljobe]

There's a java library slack-webhook

Which does a lot of the same work our SlackPayload and SlackWrapper classes are currently doing. The SlackPayload is a little problematic right now because it really mixes the responsibilities of representing a Slack message with the responsibility of how to construct that message from the TeamCity data. In looking at a way to separate those responsibilities, I stumbled upon this library which I think could simplify this plugin.

Would you be open to a pull request which switches to using it?

[eljobe]

I'm going to take the ❤️ reaction from @cy6erskunk as a "greenlight" on implementing this. You can assign the issue to me if you do that sort of bookkeeping on this project.