WASHINGTON (Reuters) – The Secret Service is investigating how an uninvited couple was admitted to U.S. President Barack Obama's White House state dinner, penetrating layers of security and mingling with VIP guests. The couple from northern Virginia was not invited to the dinner, not included on the official guest list, and were never seated at a table in the South Lawn tent where the dinner for Indian Prime Minister Manmohan Singh was held, The Washington Post reported, citing a White House official it did not identify. A Secret Service spokesman told the Post that no one was under any risk...