Issue #338: Add “priority” property to task, to take into account in scheduler.get_work

- implement handling of priority property in Task and CentralPlannerScheduler
- add documentation about task priorities
- add tests for handling priorities in CentralPlannerScheduler
Merge pull request #338 from soxofaan/feature-task-priority

Add “priority” property to task, to take into account in scheduler.get_work
Add triggering “event.core.start” when starting a task

- add event.core.start
- minor documentation fix
- add test and fine-tune other event tests a bit