(Reuters) - The death of an Illinois woman found hanged in her Texas jail cell in what local authorities said was a suicide will be investigated as thoroughly as a murder, the local district attorney said on Monday.   Sandra Bland, 28, was found dead on July 13, three days after she was arrested for assaulting an officer during a traffic stop in Waller County, about 50 miles (80 km) west of Houston, authorities have said.   The Waller County Sheriff's Office said Bland took her own life and