The founder of Foursquare always envisioned the app as a fellow traveler, dispensing relevant information unbidden — tucked-away bars as you strolled a neighborhood, sale items as you entered a boutique, or popular appetizers as you sat down for dinner at a new restaurant. But time and again, he was stymied by the massive technical challenge of building this kind of system. Finally, after 13 years of trying, Crowley and company have cracked the problem.