Attention investors: The D.C. Office of the Chief Financial Officer has set up a new web site, www.buyDCbonds.com, in order to offer approximately $270 million in income tax-backed bonds for sale on August 18 through 19. The impending bonds offer comes at the same time that CFO Natwar Gandhi announced today that the city has retained its high marks from all three ratings agencies: Standard & Poor’s once again gave D.C. a AAA rating, Moody’s…