(Reuters) - President Barack Obama hailed Libya's election on Saturday as a milestone in its post-Gaddafi democratic transition and pledged that the United States would act as a partner even as he cautioned there would still be difficult challenges ahead. "After more than 40 years in which Libya was in the grip of a dictator, today's historic election underscores that the future of Libya is in the hands of the Libyan people," Obama said in a statement after Libyans went to the polls in what was widely seen as turning the page on Muammar Gaddafi's autocratic legacy. Despite the fractious...