What is an app? A lite version of what has been called an "application" in the past? A purpose-built, single focus tool? A little of both—or something different altogether. That's a question I've been asking myself every since I downloaded Q Branch's Vesper for my iPhone. I like to mess around with a wide variety of apps for all sorts of professional reasons, so when one of my favorite bloggers (John Gruber @ Daring Fireball)…