Sanford, Florida, where 17-year-old Trayvon Martin was shot and killed by 27-year-old George Zimmerman over one month ago,  is currently host to the worst kind of attention a small town could possibly imagine.  After weeks of protests around the country, the question lingers as to whether the small town's image will be eternally marred the way that Selma or Birmingham, Alabama still evoke the civil rights movement of the 1960s.    What connection does this town have to the long history of the American civil rights movement? For answers, we turn to Isabel Wilkerson. She's a Pulitzer Prize winning journalist and author of "The Warmth of Other Suns: the Epic Story of America's Great Migration."