Agence France-Presse: US President Barack Obama on Saturday urged nations of the world to unite to resolve the most pressing global challenges — from extremism to environmental pollution to the economic crisis. In a weekly radio address timed to Easter and Passover celebrations, Obama cautioned that the world remained a dangerous place, and people must be strong and vigilant in the face of multiple threats. “But let us not allow whatever differences we have with other nations to stop us from coming …