"For me, the most important thing in your life is to live your life with integrity," Ellen DeGeneres said in 2009. She was speaking to the graduating class at Tulane University, but her wise words resonate with everyone. But Ellen's not the only celeb who's given a powerful commencement speech. Stars Amy Poehler, Robert DeNiro, and J.K. Rowling have also spoken at graduations over the years, and their w