emit stop event. make stop faster by cancelling queued invocations.

[DonutEspresso]

Previously, when stop() was called, and we weren't in the user supplied function, it would wait until right before next invocation to check before we stop.

Now, when stop is called, we check to see if we are in user supplied function, and if not, we simply clear the next scheduled invocation and stop immediately.

We also now emit a stop event on completion.

[DonutEspresso]

cc @rajatkumar @yunong

[coveralls]

Coverage remained the same at 100.0% when pulling a77f3c6 on stop-faster into 5143b2a on master.

[coveralls]

Coverage remained the same at 100.0% when pulling a77f3c6 on stop-faster into 5143b2a on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 0329a4d on stop-faster into 5143b2a on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 162ace0 on stop-faster into 5143b2a on master.