use label for upstream dependency checkbox
squash merge flatten-strings branch
Update example_top_artists.rst

Fix spelling of "exercise".
Merge pull request #336 from mikepii/flatten-strings-2

Flatten strings (squash merged)
Merge pull request #337 from pswaminathan/patch-1

Update example_top_artists.rst
Merge pull request #334 from miku/miku/updep-checkbox-label

use label for upstream dependency checkbox
Issue #338: Add “priority” property to task, to take into account in scheduler.get_work
Issue #338: add documentation about task priorities
Issue #338: added tests for handling priorities in CentralPlannerScheduler