Like a lot of people working in the apartment rental market these days, David Fitch is feeling buoyant. Demand for rental units is rising while supply remains tight. Fitch's Atlanta-based firm, Gables Residential, started raising rents two quarters ago, following rent pullbacks the prior six. Not only did occupancy rates go up on higher rents, Gables' operating costs have come down on lower property taxes and insurance fees. Gables, a private real