Tucson Shootings; Jack Dorsey

NBC Nightly News anchor Brian Williams and journalist Jon Meacham report on suspect Jared Lee Loughner and the victims of the shootings in Tucson, including Congresswoman Gabrielle Giffords. Ezra Klein joins Jon Meacham to provide his perspective on the shootings. Chairman Jack Dorsey discusses the start of Twitter and his new business venture Square, which simplifies credit card transactions.