Guardian: Ken Saro-Wiwa. Photograph: Times Newspapers Ltd/Rex Features In 1995, at a trial that resulted in his conviction and execution, the Nigerian writer and environmental activist Ken Saro-Wiwa vowed that the oil giant Shell would one day be brought to justice. That day is looming large as a New York court prepares for a trial in which the oil giant Shell stands accused of crimes against humanity over its activities in the oil-rich Niger Delta of southern Nigeria. Today, a …