Group chat for repository (Gitter)

I noticed that a lot of tasks are created just to discuss or ask some question. It would be probably better to use group chat for that, something like Gitter or Slack. I think Gitter would be better, because it stores chat logs forever.

I checked that you were asked for having Gitter chat already in #566 and #1366.

Setting up channel in Gitter is pretty straightforward. Let me know if you need any help with it.