Implement API endpoint for deleting tasks

This will always stop and delete the entire assignment, because our
architecture does not allow for stopping just one task from a multi-task
assignment.
Change return code for DELETE task to ACCEPTED

The task is not yet actually deleted when this returns, so ACCEPTED is
the more fitting return code.
Return early on bad request
Docstring for delete task
If query fits on just one line
Fix docstring for better doc generation
Outer brackets in if clause were not needed
Merge pull request #141 from guidow/delete_task_api

API endpoint for stopping the execution of tasks