Drew Houston is on top of the world. On Friday, his company, Dropbox, filed its S-1, making it Y Combinator's first startup to ever file for an IPO. The file sharing service that began as a two-person team in 2007 is now a sprawling enterprise with more than a thousand employees and a stock ticker symbol that's soon to be listed on the NASDAQ. The cloud storage giant now provides services to more than 500 million people and generates a billion dollars in annual revenue. But things weren't always easy for Houston or his company.