Fix an incorrect comment.
Change Worker#stop! to #stop, so we can use the bang method for a more immediate shutdown.
Revert "No need for a separate tasks folder now, really."

This reverts commit 2147fe8273ed95d425228fab751702d86eed157c.
Add a rask task to test Que's behavior when shutting down.
Add Que::Worker#stop! and Que.stop!, which uses custom errors to kill in-process jobs immediately.
Use Que.stop! when killing the rake tasks.
Add a spec for Que.stop! specifically.
Spec that Que.stop! doesn't blow up when there are no workers.
Stop Que in an at_exit block - don't trap signal unnecessarily.
Also shut down Que's worker pool at exit when it's running in a Rails process.
Update CHANGELOG to reflect addition of Que.stop!
Fix a comment.