Icon LLC, which took South by Southwest by storm with a home that took less than two days to build, took a major step forward this week on its mission to fight the global housing crisis with 3D printing. D.R. Horton, the largest homebuilder in the U.S., is on board along with Vulcan Capital, the investment firm of the late Microsoft co-founder and philanthropist Paul Allen and a Middle East firm that developed the world's tallest building.