The world's most towering skyscrapers have usually taken years to build, but one Chinese company thinks it only needs a fraction of that time to erect what could soon be the world's tallest building. Hunan-based Broad Sustainable Building wants to erect a 2,750-foot-tall tower in Changsha -- in only 90 days. The building, dubbed "Sky City," would beat Dubai's Burj Khalifa, currently the tallest building in the world, by 33 feet. It took five years to build the Burj Khalifa. The secr