WASHINGTON (Reuters) - President Barack Obama and first lady Michelle Obama finally celebrated their 20th wedding anniversary on Saturday - a romantic night on the town that came three days late after being pre-empted by a high-stakes presidential debate. The first couple marked the occasion with a dinner for two at the Bourbon Steak restaurant in the Four Seasons hotel in Washington's upscale Georgetown section. ....The small pool of journalists who normally accompany Obama in his motorcade went along for the ride on Saturday's date night, but did not catch a glimpse of the couple during their two-hour, 20-minute outing.