By Katharine Houreld ISLAMABAD (Reuters) - U.S. drone strikes killed a Pakistani grandmother and 18 civilian laborers last year, Amnesty International said on Tuesday, shedding new light on a major source of tension in U.S.-Pakistani relations. Islamabad publicly opposes drone attacks, saying they kill too many civilians in addition to their intended target - Islamist militants. The precise extent of human loss on the ground is unclear, however, because independent journalists and researchers have only limited access to the affected regions. ...