BZ1158818 Add "reminder" feature

here you have complete example how to use model provider to build email notification instances: https://github.com/droolsjbpm/jbpm/blob/master/jbpm-human-task/jbpm-human-task-workitems/src/main/java/org/jbpm/services/task/wih/util/HumanTaskHandlerHelper.java#L130