Twice a year, Y Combinator, a startup accelerator based in Mountain View, California, holds what it calls a Demo Day, which is a showcase of the latest batch of new companies it has nurtured. In the past, those have included Dropbox, Airbnb, and Reddit (as well as a number of companies no one remembers). Last week, at the most recent Demo Day, Eva Shang took to the stage to talk about her …