David Crosby is tweeting again, except unlike the president, you’ll actually get something of substance from the old rocker’s account. Many of Crosby’s spur-the-moment Q&A’s revolve around people asking his opinion on various artists, from Kanye West to his peers of the 60s rock and roll scene. Crosby, to his credit is always honest, giving his blunt thoughts 140 characters at a time. This…