Laura Fitton’s ascent has been staggering: In less than a year, she’s become mayor of nine different places in several different states, all without giving any speeches or kissing any babies. Instead, Fitton has gone out. A lot. And that’s allowed her to build an empire in the world of a rapidly growing Internet startup called “Foursquare,” which rewards users with points and virtual “mayorships” for checking in on their cellphones when they’re out and about. Foursquare is the brainchild of Dennis Crowley, 33, and Naveen Selvadurai, 27.