While mainstream mortgage companies have moved aggressively into the subprime market in recent years, investors think there is still room for carefully managed independent finance companies. Accredited Home Lenders, a San Diego-based mortgage banker, last week completed an initial public stock offering that netted the company $77.2 million that officials plan to use to grow the business. The company reported $4.3 billion in mortgage originations in 2003, making it the 13th largest lender in