The ouster of Citigroup (C) CEO Vikram Pandit and a stabilizing situation in Europe improve the outlook for the No. 3 U.S. bank, Sterne Agee noted in upgrading the shares to buy, from neutral. Analyst Todd Hagerman called Pandit's surprise resignation in October and replacement by Michael Corbat, a "game-changer." And while Europe remains a trouble spot, a recent upgrade in Greek debt and general improving macro situation bode well for Citi. He