Edit to stop the current execution when switching to another exercise (#304)
Merge branch 'master' into stop-execution-switch
Stop program's execution if switching to another lesson (fix #304)
Stop the demo's execution when switching from exercise or lesson
Update ChangeLog
Merge branch 'master' into stop-execution-switch
Speed up the demo's execution instead of stopping it

In order to prevent the demo's interruption from breaking the answer worlds,
only stop the current execution if it's the user's program
when switching from exercise or lesson
Merge pull request #392 from oster/stop-execution-switch

Stop execution switch