Add a TaskService that will handle single task creation as well as validation/rejection

The task service (JbpmTaskService) will make it possible to:

• create a task given a : core session, principal, task name, document, actors, and optional operation chains to launch when accepting or rejecting the tasks.
• end this task (validating or rejecting) taking care of operation chain ids set on the task.

Core events are sent in both cases, so that we can define listeners to these events.

We need to make sure that these tasks are presented on the dashboard, and they should be presented too on the document summary page (where it should be possible to accept or reject it with a comment).