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
skip requests 2.4.0 to bypass regression, see kennethreitz/requests#2192
make sure we don't handle any other kind of signal
expanding/correcting some docs while jumping back into the code
moving private methods into the internals
default attempt to 0
more docs for JobType.__init__
line fragments should probably be internal
Kill the entire subtree when killing child process

On Windows, killing a process will by default not kill that process's
child processes as well. This can be a problem when we're trying to stop
an assignment.
Kill child processes two seconds later

This gives kill() and terminate() a chance to do the right thing.