Bill Gates' favorite teacher Khan turns out thousands of videos from a converted walk-in closet in his Silicon Valley home. By David A. Kaplan, contributorAugust 24, 2010: 5:53 AM ET FORTUNE -- Sal Khan, you can count Bill Gates as your newest fan. Gates is a voracious consumer of online education. This past spring a colleague at his small think tank, bgC3, e-mailed him about the nonprofit khanaca