Software developers in Madison will gather at a local startup's offices on Saturday to participate in a "hackathon" run by Foursquare, the New York location-based social networking site. Snowshoe, a maker of mobile apps that markets an aluminum stamp designed to authenticate smartphone transactions, will host the Madison edition of the Foursquare Global Hackathon. An emerging company called hoos.in will co-host the event. At a hackathon, computer programmers, graphic designers, project managers and others collaborate intensively on software projects. Prizes will be awarded to winning projects.Foursquare allows users to post their location from