It’s not often that a Mac App Store title uses a single line description, but sometimes, that’s all that’s needed. “Slack is for making your working life simpler, more pleasant and more productive.” That’s the entire summary on the page for Tiny Speck’s Slack (Free). It’s a group communication tool made to compete with the likes of Campfire and Hipchat. We’ve been using it for a few days now, and…