Got room for an exchange student? When a 15-year-old Nigerian girl paid the San Francisco-based nonprofit organization, Aspect Foundation, thousands of dollars to place her in an American home, the last thing she expected was to to be sent to a soon-to-be-condemned house strewn with dog feces. Other students placed in Pennsylvania homes by the same organization never expected to wind up in the hospital for malnutrition and dehydration, live with ex-convicts, or be forced to eat sardines for dinn