(CBS News) The Labor Day weekend marks the unofficial start of the presidential campaign season, and many of the leading candidates for the Republican nomination have started to take shots at each other, not just at President Obama, reports CBS News political correspondent Jan Crawford. Several jabbed at Texas Gov. Rick Perry, perceived by many pundits as the latest frontrunner. That list included Sarah Palin - who's managing to keep speculation alive about whether she's going to toss her hat into the GOP ring. At a Tea Party rally in Indianola, Iowa, the former Alaska governor and Republican vice presidential...